I don't think Your problems are CPU-related.
1. It may be vid card, but not its speed but capabilities to display overlayed video. Some old cards have great problems with it, for example i815 integrated graph chip. My brother had one
, it sometimes needed even 15s to switch to overlay, and some codecs/players where not able to use overlay at all.He solved his video-watching problems by buying cheapest GF4MX (he had Celeron 1200 CPU). BTW my VERY old CL5446-based video card (it was PCI graphics card with integrated TV-tuner) did not had any problems with displaying overlays. My later TNT M64 had, but moving to GF2MX helped. I don't think any new card should have any problems with overlay now.
2. Problem may be with codec itself. I use ffdshow codec to decode almost everything, and once I had problems with some clips encoded with newest versions of divx/xvid (video was very choppy, like showing every second frame). Downloading newest version of ffdshow helped
. If you compare two machines by playing video clips, make sure they use the same codes, cofigured in the same way (deblocking and deringing are very CPU-intensive).
BTW my P4 2.8 Northwood is able to decode 1024x576 (biggest avi I got
video with ffdshow postprocessing set to max without lagging, but ffdshow _is_ very fast. I know this becouse on my sister's Celeron 500 (with original Creative's GeForce 256 DDR
regular DivX codec lags terribly with hi-res video, and ffdshow does just fine.