Using 1.5.2 on the OLD launcher, I got around 900fps, highest graphics settings. This was on a server.
On the same server, but using the new launcher and 1.5.2, I am only getting 75fps.
A friend of mine is having a very similar problem, except his GPU is not as good so Minecraft is rendered unusable/playable. He uses a Mac, which also shows this is not hardware/software related.
I know this is a launcher issue, as it never happened before. I have also checked the integrated graphics are not selected instead. It has the same problem on 1.6.1 too, further proving it is not the game that is causing this issue.
I know 75fps is still ok, but under some circumstances it can seriously lag, and only since using this launcher.
If anyone has a copy of the OLD launcher online for download I would be extremely grateful, just for use until this is fixed.