All mob movement is laggy. The player is entirely unaffected. Examples include striking a zombie or skeleton with a sword and it flies through the air slowly and in a stuttered fashion. Another example is that creepers take about five seconds, if not more, to explode. Skeleton arrows are unaffected, however, and items are unaffected as well. Passive mobs move in a laggy way too. Particularly egregious is that I can jump a horse, dismount in mid-air, and fall to the ground well before the horse does.

This only happens in one world, not in ones created before or after. I did try transferring the save data to another computer, and the same problems were present there, so it is computer-independent. I have attached the files I think are sufficient to reproduce the problem, but just in case, here's a link to the whole saved game folder (zipped, on Google Drive): https://drive.google.com/file/d/0By56vQbpf7Hdc2p0aWM5T1N3ZWM/view?usp=sharing