There is a bug with how Linux does it's CPU speed scaling. For some reason it won't scale the cpu up to make graphics run faster.
If you set the cpu speed at maximum or choose the 'performance' governer then it will make things quite a bit faster. For example in ManiaDrive I average around 50-70fps with my cpu at 800mhz and 150-190 when its at 2.0ghz.
However even with that your still going to see some performance drop. Like I said the only thing that I've found that works well so far is Fedora 11 beta... and even then it took quite a bit of time before they got it working well. As the new code paths mature I have no doubt that they will be able to exceed the capabilities of the old versions of the drivers.. especially when the drivers gain more capabilities.
The trade off here is that with a composited desktop OpenGL applications prior to UXA/GEM/DRI2/etc was completely unusable...