[mythtv] Need help diagnosing slow XvMC problem

[Jerry Rubinow]

When I play back content using XvMC, it plays back slower than real
time, both HD and SD, cpu is not maxed out.  I haven't been able to
figure out why, but it seems like it might be a Myth problem (but
maybe not?).

Here's what I have:

Separate front end/back end.  Front end is a 2.8Ghz P4 with a GeForce 5200 PCI.
Running FC4 setup per Jarod's guide.
Myth SVN as of about a week ago.
Nvidia driver version 7667.

Here's what I've tried:

Tried with and without OpenGL sync.
Tried Various Nvidia drivers up the the current 81xx.
Tried playing same video files over nfs using mplayer with xvmc and
they have no problem, so that seems to eliminate disk, network, or
graphics card bandwidth problems.
No deinterlacing going on.

When it plays inside myth, everything is slow-mo.  Not a stuttering
problem.  CPU usage is down below 20%.  If I turn off XvMC, SD plays
back fine, but HD maxes out the CPU.  However, I hadn't been
optimizing for P4 (d'oh), so that can probably be improved somewhat. 
But if I could get the XvMC thing working that would be a much better
solution, since I'd have room to spare for deinterlacing and whatnot.

All suggestions welcome.  I'll post whatever logs would be most
helpful to diagnose this, just let me know which ones.

-Jerry