As some other people mentioned in this post:
I with them get really high CPU of Xorg whenever I play one of my 720p recordings on my laptop, desktop, mythbox and any machine really running an nvidia card in both from the mythfrontend or mplayer.
What is up with this issue? The only way to stop it by disabling vsync but then you get ugly tearing which is not acceptable. I have had this issue for a very long time only now to find out what was causing it. On faster machines or dual core machines this issue doesn't cause too many problems other than the fact it uses up way more CPU than it should but on slower machines it will cause the video to get lagged.
I only have this issue on 720p recordings which I believe is because they are 60FPS the framerate as the refresh rate..