It's pretty easy to explain.
While using the old launcher, 1.5.1 was at a very convenient FPS of about 150. However, using the new launcher cut my FPS down to about 20 in regular worlds, while on flatlands it levelled at 35.
I can see a connection here. The new launcher closes itself and opens a new window, while the old launcher didn't. It seems that by opening a new window, the gameplay is slowed down a lot. This has occurred with the Feed the Beast modpack as well as MagicLauncher, which both have launchers that close after launch and open a new window to play Minecraft in.
This is really inconvenient and I don't want to play Minecraft where by pressing F5 twice, my FPS will drop to 5.
EDIT: Somehow it got fixed. Or maybe Mojang did something that fixed it, now getting a 200 FPS on flatlands. Thanks version 0.2!