Trying this bug report thing again. This is a major issue. I have screenshots to support the report this time. In the newest updates 1.7 prerelease and 1.7.1 I am still experiencing major FPS issues. The memory used rarely goes above about 15%. In the last version that I was able to play (13w39b) I don't have this issue at all.

One thing I noticed is that in the version that works (13w39b), my FPS stays about the same when paused and when in game. In the version that doesn't work I have low FPS in game, but in the pause menu, my FPS goes up to what it usually is (or better) in the version of the game that works.

What has changed in Minecraft to cause such a drastic issue with my FPS?