World suddenly crashed / slow

[LordFlowerpot]

After a tekkit classic world had glitched out, I decided to create a new one. It worked perfectly fine without lag. I mined, found diamonds and started building a home. Now, when I try to play this world, it either crashes or is really slow. By reallt slow I mean that when I move my mouse it takes like 10 seconds to react. When I press esc it opens up the menu after a couple of seconds and that works perfectly fine. I don't know what to do now, I really really don't want to lose this world. Tell me if you need any files.