FPS Problems on highly expensive and good computer

I was having a friend over, (He's really new to Minecraft, so expect unrealistic things to be said), and he was in Default terrain gen when his FPS rates plumited from a decent 60 to a terrible 5! He was like "Evan, make it not lag!!!" Details about computer:
Bought in 2011, an Apple iMac, haven't used Boot Camp, 1TB drive, and 12GB of RAM, and 4GB dedicated to Minecraft. I don't know how to replicate it. It's not the newer, thinner Macs, it's 1 inch thick on the side and 2 inches thick in the middle. It is also the model with the gray square at the bottom.