Why is Minecraft so laggy on this Mac?

Answer 1

It depends,

Are you playing online on someone elses server, they or you may have a low end internet bandwidth. If the server has a lower end DSL of 1.5Mbs let's say, then it's only good for 2 to 4 people before it starts dropping connections or gets laggy.

If your hosting the server and playing the game then it can effect a lower end or older computer. Try upping the ram on the server.

If your just playing single player make sure you don't have any applications running in the background, minecraft is a pretty hardware intensive game. Also make sure any automatic updates for any applications are set for the middle of the night so it doesn't interfere with your game, downloading updates and trying to play an online game is a bummer sometimes.

When was the last time you shut down the computer for a few minutes, it will reset the ram and other caches so may free up some things.