I have a custom mod pack which includes OreSpawn. It runs fairly well at an acceptable framerate, except when entering the brown ant dimension, even with the render distance set to 4, it is unplayably laggy and freezes up. I have tested the red ant, crystal and moth dimensions and they run fine. I was thinking it could be to do with Treecapitator, as there a lot of trees in the dimension, but without Treecap the issue was still there.
When you load the world, it seems fine for about 30 seconds, and then starts to lag. When it lags it will freeze completely for as long as five seconds, render 1 frame, wait another five seconds, render a frame, etc. Every so often it runs perfectly smoothly for about five seconds and then goes back to freezing.
Mods are as follows: Forge 10.13.4.1558-1.7.10, Better Furnaces, Damage Indicators, Lucky Block Rainbow, Battlegear 2, Animation API, Backpack, BackTools, Chocolate Quest, Code Chicken Core, Enchanting Plus, Explosives++, Rotten Flesh to Leather, Gravity Gun, Hardcore Ender Expansion, Hats, iChunUtil, Infernal Mobs, Iron Chests, Kingdoms of the Overworld, Loot++, Loot Bags, Mantle, Minecraft Animated, Monster Hunter Frontier Craft, Minespresso, M-Ore, Morph, Mr Crayfish Furniture, More Swords, Much More Spiders, Mutant Creatures, Not Enough Items, Obsidian Utilities, OptiFine, Orespawn, Pandora's Box, Power Tools, Rainbow World, Rival Rebels, Poop, Tinker's Construct, Special Armor, Torched, Trail Mix, Villager's Nose, Xaero's Minimap
All mods are fully updated as of today, with the exception of Minecraft Animated which uses a super old version required by Monster Hunter Frontier Craft.