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Help! Whenever I try to go into Minecraft it lets me play for a few seconds then crashes. It says [WARNING] Cant Keep Up! Did the system time change, or is the server overloaded. The only mod I am using is Optifine for 1.9. I'm not using any texture packs or shaders. Please help!!!

Yes, my computer is slow. I just started playing minecraft 1.9 but whenever I try to go into my singleplayer world minecraft stops working and crashes. Here is the error

Mar 27, 2016 9:13:48 AM net.minecraft.launchwrapper.LogWrapper log
INFO: Loading tweak class name optifine.OptiFineTweaker
Mar 27, 2016 9:13:48 AM net.minecraft.launchwrapper.LogWrapper log
INFO: Using primary tweak class name optifine.OptiFineTweaker
Mar 27, 2016 9:13:48 AM net.minecraft.launchwrapper.LogWrapper log
INFO: Calling tweak class optifine.OptiFineTweaker
OptiFineTweaker: acceptOptions
OptiFineTweaker: injectIntoClassLoader
OptiFine ClassTransformer
OptiFine URL: file:/C:/Users/Catherine/AppData/Roaming/.minecraft/libraries/optifine/OptiFine/1.9_HD_U_B1/OptiFine-1.9_HD_U_B1.jar
OptiFine ZIP file: C:\Users\Catherine\AppData\Roaming\.minecraft\libraries\optifine\OptiFine\1.9_HD_U_B1\OptiFine-1.9_HD_U_B1.jar
OptiFine Version: OptiFine_1.9_HD_U_B1
Minecraft Version: 1.9
OptiFineTweaker: getLaunchArguments
OptiFineTweaker: getLaunchTarget
Mar 27, 2016 9:13:49 AM net.minecraft.launchwrapper.LogWrapper log
INFO: Launching wrapped minecraft {net.minecraft.client.main.Main}
Completely ignored arguments: [--nativeLauncherVersion, 301]
[09:13:52] [Client thread/INFO]: Setting user: Moon_Light_2900
[09:13:53] [Client thread/INFO]: (Session ID is <censored>)
[OptiFine] (Reflector) Class not present: net.minecraftforge.common.MinecraftForge
[OptiFine] (Reflector) Class not present: net.minecraftforge.common.ForgeHooks
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.MinecraftForgeClient
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.ForgeHooksClient
[OptiFine] (Reflector) Class not present: net.minecraftforge.fml.common.FMLCommonHandler
[OptiFine] (Reflector) Class not present: net.minecraftforge.fml.client.FMLClientHandler
[OptiFine] (Reflector) Method not present: asv.getSkyRenderer
[OptiFine] (Reflector) Method not present: asv.getCloudRenderer
[OptiFine] (Reflector) Method not present: asv.getWeatherRenderer
[OptiFine] (Reflector) Method not present: aht.countEntities
[OptiFine] (Reflector) Method not present: aht.getPerWorldStorage
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.IRenderHandler
[OptiFine] (Reflector) Class not present: net.minecraftforge.common.DimensionManager
[OptiFine] (Reflector) Class not present: net.minecraftforge.event.world.WorldEvent$Load
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.event.RenderItemInFrameEvent
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.event.GuiScreenEvent$DrawScreenEvent$Pre
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.event.GuiScreenEvent$DrawScreenEvent$Post
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.event.EntityViewRenderEvent$FogColors
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.event.EntityViewRenderEvent$FogDensity
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.event.EntityViewRenderEvent$RenderFogEvent
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.event.EntityViewRenderEvent$CameraSetup
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.event.RenderLivingEvent$Pre
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.event.RenderLivingEvent$Post
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.event.RenderLivingEvent$Specials$Pre
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.event.RenderLivingEvent$Specials$Post
[OptiFine] (Reflector) Class not present: net.minecraftforge.fml.common.eventhandler.EventBus
[OptiFine] (Reflector) Class not present: net.minecraftforge.fml.common.eventhandler.Event$Result
[OptiFine] (Reflector) Class not present: net.minecraftforge.event.ForgeEventFactory
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.event.RenderBlockOverlayEvent$OverlayType
[OptiFine] (Reflector) Class not present: net.minecraftforge.event.world.ChunkWatchEvent$UnWatch
[OptiFine] (Reflector) Method not present: ajt.getBedDirection
[OptiFine] (Reflector) Method not present: ajt.isBed
[OptiFine] (Reflector) Method not present: ajt.isBedFoot
[OptiFine] (Reflector) Method not present: ajt.hasTileEntity
[OptiFine] (Reflector) Method not present: ajt.canCreatureSpawn
[OptiFine] (Reflector) Method not present: ajt.addHitEffects
[OptiFine] (Reflector) Method not present: ajt.addDestroyEffects
[OptiFine] (Reflector) Method not present: ajt.isAir
[OptiFine] (Reflector) Method not present: ajt.canRenderInLayer
[OptiFine] (Reflector) Method not present: ajt.getExtendedState
[OptiFine] (Reflector) Field not present: rr.captureDrops
[OptiFine] (Reflector) Field not present: rr.capturedDrops
[OptiFine] (Reflector) Method not present: rr.shouldRenderInPass
[OptiFine] (Reflector) Method not present: rr.canRiderInteract
[OptiFine] (Reflector) Method not present: rr.shouldRiderSit
[OptiFine] (Reflector) Method not present: apv.shouldRenderInPass
[OptiFine] (Reflector) Method not present: apv.getRenderBoundingBox
[OptiFine] (Reflector) Method not present: apv.canRenderBreaking
[OptiFine] (Reflector) Method not present: ado.onEntitySwing
[OptiFine] (Reflector) Method not present: ado.shouldCauseReequipAnimation
[OptiFine] (Reflector) Method not present: ado.showDurabilityBar
[OptiFine] (Reflector) Method not present: ado.getDurabilityForDisplay
[OptiFine] (Reflector) Method not present: rl.isCurativeItem
[OptiFine] (Reflector) Method not present: aef.getRecordResource
[OptiFine] (Reflector) Method not present: bvs$b.preDraw
[OptiFine] (Reflector) Method not present: bvs$b.postDraw
[OptiFine] (Reflector) Class not present: net.minecraftforge.fml.common.asm.transformers.BlamingTransformer
[OptiFine] (Reflector) Class not present: net.minecraftforge.fml.relauncher.CoreModManager
[OptiFine] (Reflector) Class not present: net.minecraftforge.fml.client.SplashProgress
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.model.pipeline.LightUtil
[OptiFine] (Reflector) Class not present: net.minecraftforge.common.property.IExtendedBlockState
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.ItemModelMesherForge
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.model.ModelLoader
[OptiFine] (Reflector) Class not present: net.minecraftforge.client.model.Attributes
[OptiFine] (Reflector) Class not present: mods.betterfoliage.client.BetterFoliageClient
[OptiFine] (Reflector) Method not present: aig.getWaterColorMultiplier
[OptiFine] (Reflector) Class not present: net.minecraftforge.fml.client.registry.RenderingRegistry
[OptiFine] (Reflector) Method not present: bpm.preDrawBatch
[OptiFine] (Reflector) Method not present: bpm.drawBatch
[OptiFine] (Reflector) Method not present: boq.handleItemState
[OptiFine] (Reflector) Class not present: net.minecraftforge.common.ForgeModContainer
[09:13:58] [Client thread/INFO]: LWJGL Version: 2.9.4
[OptiFine]
[OptiFine] OptiFine_1.9_HD_U_B1
[OptiFine] Sun Mar 27 09:13:58 PDT 2016
[OptiFine] OS: Windows 8.1 (amd64) version 6.3
[OptiFine] Java: 1.8.0_25, Oracle Corporation
[OptiFine] VM: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
[OptiFine] LWJGL: 2.9.4
[OptiFine] OpenGL: Intel(R) HD Graphics, version 4.0.0 - Build 10.18.10.3925, Intel
[OptiFine] OpenGL Version: 4.0
[OptiFine] OpenGL Fancy fog: Not available (GL_NV_fog_distance)
[OptiFine] Maximum texture size: 8192x8192
[Shaders] ShadersMod version: 2.4.12
[Shaders] OpenGL Version: 4.0.0 - Build 10.18.10.3925
[Shaders] Vendor: Intel
[Shaders] Renderer: Intel(R) HD Graphics
[Shaders] Capabilities: 2.0 2.1 3.0 3.2 4.0
[Shaders] GL_MAX_DRAW_BUFFERS: 8
[Shaders] GL_MAX_COLOR_ATTACHMENTS_EXT: 8
[Shaders] GL_MAX_TEXTURE_IMAGE_UNITS: 16
[Shaders] Load ShadersMod configuration.
[Shaders] No shaderpack loaded.
[OptiFine] Checking for new version
[09:14:00] [Client thread/INFO]: Reloading ResourceManager: Default
[OptiFine] *** Reloading textures ***
[OptiFine] Resource packs: Default
[OptiFine] Version found: A3_pre
[09:14:02] [Sound Library Loader/INFO]: Starting up SoundSystem...
[09:14:02] [Thread-6/INFO]: Initializing LWJGL OpenAL
[09:14:02] [Thread-6/INFO]: (The LWJGL binding of OpenAL. For more information, see http://www.lwjgl.org)
[09:14:07] [Thread-6/INFO]: OpenAL initialized.
[09:14:07] [Sound Library Loader/INFO]: Sound engine started
[OptiFine] Mipmap levels: 4
[OptiFine] Multitexture: false
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/0_glass_white/glass_pane_white.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/0_glass_white/glass_white.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/10_glass_purple/glass_pane_purple.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/10_glass_purple/glass_purple.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/11_glass_blue/glass_blue.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/11_glass_blue/glass_pane_blue.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/12_glass_brown/glass_brown.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/12_glass_brown/glass_pane_brown.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/13_glass_green/glass_green.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/13_glass_green/glass_pane_green.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/14_glass_red/glass_pane_red.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/14_glass_red/glass_red.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/15_glass_black/glass_black.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/15_glass_black/glass_pane_black.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/1_glass_orange/glass_orange.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/1_glass_orange/glass_pane_orange.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/2_glass_magenta/glass_magenta.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/2_glass_magenta/glass_pane_magenta.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/3_glass_light_blue/glass_light_blue.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/3_glass_light_blue/glass_pane_light_blue.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/4_glass_yellow/glass_pane_yellow.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/4_glass_yellow/glass_yellow.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/5_glass_lime/glass_lime.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/5_glass_lime/glass_pane_lime.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/6_glass_pink/glass_pane_pink.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/6_glass_pink/glass_pink.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/7_glass_gray/glass_gray.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/7_glass_gray/glass_pane_gray.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/8_glass_silver/glass_pane_silver.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/8_glass_silver/glass_silver.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/9_glass_cyan/glass_cyan.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/9_glass_cyan/glass_pane_cyan.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/bookshelf.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/glass.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/glasspane.properties
[OptiFine] ConnectedTextures: mcpatcher/ctm/default/sandstone.properties
[OptiFine] Multipass connected textures: false
[09:14:11] [Client thread/INFO]: Created: 1024x512 textures-atlas
[OptiFine] *** Reloading custom textures ***
[09:14:18] [Realms Notification Availability checker #1/INFO]: Could not authorize you against Realms server: Invalid session id
[09:14:22] [Client thread/ERROR]: Exception reading C:\Users\Catherine\AppData\Roaming\.minecraft\saves\FIRST SURVIVAL WORLD!!!\level.dat
java.util.zip.ZipException: Not in GZIP format
at java.util.zip.GZIPInputStream.readHeader(GZIPInputStream.java:165) ~[?:1.8.0_25]
at java.util.zip.GZIPInputStream.<init>(GZIPInputStream.java:79) ~[?:1.8.0_25]
at java.util.zip.GZIPInputStream.<init>(GZIPInputStream.java:91) ~[?:1.8.0_25]
at dx.a(SourceFile:24) ~[dx.class:?]
at azg.a(SourceFile:82) [azg.class:?]
at azg.c(SourceFile:66) [azg.class:?]
at azd.b(SourceFile:57) [azd.class:?]
at bho.e(SourceFile:35) [bho.class:?]
at bho.<init>(SourceFile:28) [bho.class:?]
at bhm.b(SourceFile:43) [bhm.class:?]
at bfb.a(SourceFile:367) [bfb.class:?]
at bcf.a(SourceFile:894) [bcf.class:?]
at bfi.a(SourceFile:237) [bfi.class:?]
at bfb.a(SourceFile:341) [bfb.class:?]
at bfi.a(SourceFile:500) [bfi.class:?]
at bfb.k(SourceFile:404) [bfb.class:?]
at bfb.p(SourceFile:381) [bfb.class:?]
at bcf.t(SourceFile:1575) [bcf.class:?]
at bcf.av(SourceFile:962) [bcf.class:?]
at bcf.a(SourceFile:392) [bcf.class:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [Main.class:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_25]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_25]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_25]
at java.lang.reflect.Method.invoke(Method.java:483) ~[?:1.8.0_25]
at net.minecraft.launchwrapper.Launch.launch(Launch.java:131) [launchwrapper-1.7.jar:?]
at net.minecraft.launchwrapper.Launch.main(Launch.java:27) [launchwrapper-1.7.jar:?]
[09:14:22] [Client thread/ERROR]: Exception reading C:\Users\Catherine\AppData\Roaming\.minecraft\saves\FIRST SURVIVAL WORLD!!!\level.dat_old
java.util.zip.ZipException: Not in GZIP format
at java.util.zip.GZIPInputStream.readHeader(GZIPInputStream.java:165) ~[?:1.8.0_25]
at java.util.zip.GZIPInputStream.<init>(GZIPInputStream.java:79) ~[?:1.8.0_25]
at java.util.zip.GZIPInputStream.<init>(GZIPInputStream.java:91) ~[?:1.8.0_25]
at dx.a(SourceFile:24) ~[dx.class:?]
at azg.a(SourceFile:82) [azg.class:?]
at azg.c(SourceFile:74) [azg.class:?]
at azd.b(SourceFile:57) [azd.class:?]
at bho.e(SourceFile:35) [bho.class:?]
at bho.<init>(SourceFile:28) [bho.class:?]
at bhm.b(SourceFile:43) [bhm.class:?]
at bfb.a(SourceFile:367) [bfb.class:?]
at bcf.a(SourceFile:894) [bcf.class:?]
at bfi.a(SourceFile:237) [bfi.class:?]
at bfb.a(SourceFile:341) [bfb.class:?]
at bfi.a(SourceFile:500) [bfi.class:?]
at bfb.k(SourceFile:404) [bfb.class:?]
at bfb.p(SourceFile:381) [bfb.class:?]
at bcf.t(SourceFile:1575) [bcf.class:?]
at bcf.av(SourceFile:962) [bcf.class:?]
at bcf.a(SourceFile:392) [bcf.class:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [Main.class:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_25]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_25]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_25]
at java.lang.reflect.Method.invoke(Method.java:483) ~[?:1.8.0_25]
at net.minecraft.launchwrapper.Launch.launch(Launch.java:131) [launchwrapper-1.7.jar:?]
at net.minecraft.launchwrapper.Launch.main(Launch.java:27) [launchwrapper-1.7.jar:?]
[09:14:28] [Server thread/INFO]: Starting integrated minecraft server version 1.9
[09:14:28] [Server thread/INFO]: Generating keypair
[09:14:29] [Server thread/INFO]: Preparing start region for level 0
[09:14:30] [Server thread/INFO]: Preparing spawn area: 0%
[09:14:31] [Server thread/INFO]: Preparing spawn area: 21%
[09:14:32] [Server thread/INFO]: Preparing spawn area: 53%
[09:14:33] [Server thread/WARN]: Keeping entity Sheep that already exists with UUID 1dc19feb-9129-49f6-b1b5-b5e94ed0cb07
[09:14:33] [Server thread/WARN]: Keeping entity Bat that already exists with UUID 9f11c1a9-bedc-410c-93d5-4dea579290c9
[09:14:33] [Server thread/WARN]: Keeping entity Sheep that already exists with UUID d02cb105-a551-4796-8a80-1553848c8676
[09:14:33] [Server thread/WARN]: Keeping entity Bat that already exists with UUID c94d640b-0b31-42f8-8694-f94d3eb5e01d
[09:14:33] [Server thread/WARN]: Keeping entity Skeleton that already exists with UUID e09ea0b5-9bc8-4342-8a0c-deda57a39aea
[09:14:33] [Server thread/WARN]: Keeping entity Bat that already exists with UUID bf43d428-cdc3-4c8c-8d27-0ce6ea7ff8a0
[09:14:33] [Server thread/INFO]: Preparing spawn area: 83%
[09:14:33] [Server thread/WARN]: Keeping entity Bat that already exists with UUID 5bfafe66-b867-4b7f-a539-4daf9c919cf1
[09:14:33] [Server thread/WARN]: Keeping entity Bat that already exists with UUID 8bb3d8f4-0318-4eba-80e6-6ab7b637ed88
[09:14:34] [Server thread/INFO]: Changing view distance to 3, from 10
[09:14:37] [Server thread/INFO]: Moon_Light_2900[local:E:af004576] logged in with entity id 217 at (183.75070416383318, 66.0, 111.94327391975982)
[09:14:37] [Server thread/INFO]: Moon_Light_2900 joined the game
[09:14:39] [Server thread/INFO]: Saving and pausing game...
[09:14:39] [Server thread/INFO]: Saving chunks for level 'New World'/Overworld
[09:14:40] [Server thread/INFO]: Saving chunks for level 'New World'/Nether
[09:14:40] [Server thread/INFO]: Saving chunks for level 'New World'/The End
[09:14:40] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2777ms behind, skipping 55 tick(s)

This has never happened before it started happening when I got optifine 1.9.

Quote from MCOfficer»

if this is a new world (which i assume), your world generator seems to be corrupted itself (so it generates corrupted worlds). try re-installing minecraft completely (delete the entire folder C:\Users\Catherine\AppData\Roaming\.minecraft\ , then execute your minecraft launcher again)

No you don't - the launcher will automatically reinstall any file that doesn't match the checksum on Mojang's servers. In fact, you can't even modify the jar anymore without some trickery since they added a checksum and file size to the json file for every single file used by the game (the launcher will even delete the file if you are offline, preventing you from playing; making it read-only will similarly fail, only by removing these arguments can you mod the jar):

MCL-4334 Minecraft gets reinstalled everytime you launch it

At the very most you only need to reinstall Optifine.

Also, we need the crash report (one should be present), not game output, to see what is causing the crash; the "error" you mention is not related to any sort of crash and appears for pretty much anybody, at least after loading a world, nor should persistent lag cause the game to crash, only run slower. The other error (corrupted level.dat_old) may have been caused by a previous crash (it is very common for level.dat to get corrupted by a crash; level.dat_old is a backup made, which may have been corrupted while being copied)