I just got Vegas 4 for my home machine (Dual 1.7Ghz Athlon with ADS Pyro) but noticed the same thing with the Vegas demo on a machine at work (Dual 2.8Ghz Xeon with built-in firewire). When in the capture app, the GUI becomes horrendously sluggish.. Pushing the 'play' button doesn't do anything for several seconds, trying to drag the shuttle bar around is...interesting... the timecode will run for a second and then freeze up for a few seconds, etc. It all seems to work OK but it's terribly annoying.
Is this normal? And if it is, are there any 3rd party capture apps I could use instead? Because this is nearly unusable. Totally frustrating.