Sounds like hardware acceleration is no enabled.
Below is the setting for fastest mpeg4 decoding.
1) Use display driver from card manufacturer, not the default one from windows. Default/Microsoft drver = slow
2) Turn on all hardware video acceleration in DirectX. You can access that from: start menu -> run -> type in "dxdiag"
3) Enable overlay, use YV12 decoding, if YV12 not avavilable use YUV2.
In ffdshow -> enable "use overlay mixer", enable YV12 and YUV2 output.
In DIVX decoder -> enable "use overlay extended mode" and "use YUV extended mode".
Remarks:
a) Overlay mixer can cause trouble in WMP9. if so, switch to WMP6.4
b) XVID decoder is buggy, don't use it
