RP2 + Cave World lags because of how RP2 generates Marble. Once generation is complete, you don't have any further problems.

This is a known bug, which has the potential for crashing your game. Unfortunately, there's not a whole lot one can do about it. Best thing, if you don't use NEI, is to go into an unexplored world with minimal and sacrificial equipment. My typical exploration setup is about a half stack of torches, my link book back to the Overworld, bread or melons, and stone tools. If I crash and it punts me back to spawn, no big harm done.