In my experience this is not the case. I am using the new AMD drivers (I belive) on Ubuntu 11.04 and the performance is terrible. Alt-tabbing to switch between windows even incurs a second+ delay. I reverted back to the open source one and it at least functions.
I had this problem until I disabled vertical refresh. Based on posts in the ubuntu forums this appears to solve most people's problems with the AMD drivers.
I think 11.04 is just slow. I installed this on my nvidia ion-based desktop (that I use for watching TV, etc.), and it's much slower than the previous revision. Everything is glacial. I think I'm just going to replace Ubuntu with my Usual Setup (Debian), because all I do is run wget and mplayer anyway.
