Singleplayer crashes

Whenever I try to play singleplayer, Minecraft takes a very long time to load the world. Then I have severe lag. When I exit, Minecraft crashes. I have tried many things. I even re-installed Windows! (That made everything worse - it used to just crash on exit). If I play in multiplayer, everything works great. (Luckily I own a family & friends server, so it's not that hard. But it's still easier to play singleplayer). My sister has a computer, and I have a laptop. They don't have the crash. And nobody I know has had this crash. I don't have any mods.

Thanks.