When I use the new client with version 1.5.2 of minecraft, I have very low fps, but if you use the old client, I have the normal fps.
The same happens with 1.6.1.

My System:
8GB RAM
Intel Core i7-3770 3.40ghz
Intel HD Graphics 4000 (I know it's bad, but do not understand why the difference in fps in diferencites versions)

If I can help, to use the 1.6.1 version in the old client and to play as always, the appreciate.

Saludos desde México.