Xserver/i810 driver and very high CPU usage

I'm working with XVIDEO for HD with an i945G via the i810 driver.
I have a patch from alanh that allocate memory for XVIDEO playback of
the 1280x720 HD frames.

When the display is 1280x1024 I see smooth playback and low CPU use.
Xine uses 50% and X hardly shows up in top on a 3GHz P4.
But when the display is 1280x768 the X CPU usage hits 50%, the other
50% is in Xine and playback is poor.

1) Anyone know why this might be?
2) How do I go about debugging this?

Barry