I am using intel GM965 graphics with Ubuntu 9.0.4 and the intel driver version 2.6.3 (though the same results occur with 2.7.x as far as I can see).
Steps to reproduce:
0. Specify UXA acceleration in xorg.conf
1. Open a terminal and run "top".
2. Open another terminal and do, for example
$ zenity --progress --percentage=90 --width=600
so as to create a little bit of graphics activity.
3. Observe that xorg is using (perhaps) 19% of one core.
Repeating the experiment with EXA shows only about 6-8% usage. Other simple things similar to progress bars use even more CPU under UXA. I noticed this first when using the volume control in pavucontrol: the bar indicating volume output levels moves back and forth and uses 50% of one core. My cpu is a Core 2 duo.
When I run that on my system, the CPU is entirely idle. zenity doesn't appear to do rendering over time. From playing mp3s with pavucontrol open, Xorg's CPU usage jumps to 20%. Does this match your experience with xf86-video-intel master? It should be the same performance as EXA now since:
Author: Carl Worth <firstname.lastname@example.org>
Date: Fri May 29 15:34:20 2009 -0700
UXA: Rasterize trapezoids to system memory, not a pixmap
feedback timeout, and the change should have fixed the issue.