^^ No performance drop, if you have 256+ Mb of Ram
and it depends on Desktop Environment too..Here is a short explanation of Memory /CPu usage with compiz
Compiz= Desktop Effects that you are seeing .. the other mentioned are the Desktop Environment
Compz + XFCE < Compiz + Gnome < Compiz + KDE
Note: Those SS are posted when I was on 256 Mb Ram and Onboard GPU [though I still have same MB
] .. Also new compiz fusion is much better than older 1 . So if you still have doubts check compiz fusion 0.7x series
For newer SS check this
http://www.techenclave.com/graphics-and-eyecandy/the-desktop-thread-97530.html