I can't play single player on Minecraft. ;(

Whenever I launch Minecraft, I go to Single Player and create a world or try to play on a world I already have, It keeps saying "Loading Chunks" for about 7 minutes then when it finally gets it, I only get like 30 fps and it keeps going lower and lower until it gets to 1 fps and then I can't move and I crashes. This only happens to me in single player and I've already tried deleting Minecraft and downloading it again but it doesn't change anything. I think I does it on both survival and creative.