Xfree86.0 high CPU in 1280x768

I'm seeing the X process take a lot more CPU time to run any workload in
1280x768 compared to a standard VGA mode like 1280x1024.

For example running a text scrolling app (lots of XCopyArea calls) I see the
following CPU usage figures:

1280x1024:  6.75%
1280x768:  10.53%

top shows that the X process is the only one to show a change between 
the two modes.

With XVIDEO apps the difference is from 50% to 70% to play a movie.

This happens with the  i810 driver and the via driver so  I don't think 
its a driver
specific issue. I think that X is changing its behavior.

Is it possible that X has turned off it acceleration in 1280x768 mode?

What can I look at to find out what the problem is?

Barry