possible that your hard disk is fragmented, a disk defrag might help or just that you have to many programs running or a slow computer. I would say that mods and other add-ons might be doing this, more files in the minecraft.jar file will sometimes cause this. you might also try giving minecraft more ram google it.
I'll do a disk defrag, yeah. But the thing is, I'm using the exact same mods I've always been using, and this is the first time this happened. So I don't know if it's the mods that are causing it.
Edit: Disk defrag done, rebooted my pc. Nothing changed.
All my sound drivers are updated and Java is updated too. I don't know what's causing this sudden lag, but it's really annoying.
Edit: Disk defrag done, rebooted my pc. Nothing changed.