OpenGL Vsync on MythTV
I had enabled Open Vsync on our MythTV frontend during the initial configuration with the belief that there couldn't be any harm in adding an additional method of vertical synchronization. I changed my mind when I began noticing strange delays in the video playback on my NVidia FX 5200 with XVMC. After changing channels, there would be about 3-5 glitches in video playback for 30 seconds, followed by okay playback. I was okay with it until we began seeing much more dramatic choppiness in playback coupled with higher CPU utilization (typically 18-20%, but as high as 70%).
I searched the web for the symptoms and saw a lot of people reporting problems with video playback when OpenGL Vsync is enabled (here and here). Since disabling OpenGL Vsync, I've not seen any delays in playback. I might re-enable UseEvents in /etc/X11/xorg.conf, but it's currently set to False.