When sand and gravel falls, it falls very "chunkily". It used to be smooth, but now the frame rate goes down to something like 3 FPS for falling blocks.

This happens for minecarts as well--they seem to "jiggle" while I'm riding them. Their frame rate is reduced as well.

This does not affect the overall frame rate, just the frame rate for the falling blocks and minecarts.

EXTRA INFO: This only happened on my vanilla Minecraft server in Survival mode. I'm not sure about single player.