The Xv support only works when your XFree86 server supports your
particular graphics card. If it works, then the frames per second
value will be much higher.

I don't have a complete list, but at least the Matrox Millenium G200
can be used with XFree86 V. 4.0.1. The Matrox Millenium II does not
work. Check the XFree86 website and mailing list for more information and
support.

With the G200 and XFree86 4.0.1, the screen depth must be set to
either 16 or 24 bits. If using 24 bits, the screen size must be no
more than 1024 * 768.