There may be nothing wrong. Fast renders usually produce inferior quality results. Have you compared the outputs of different solutions?
Have you checked that you didn’t accidentally set the opacity to less than 100%? It’s too easy to bump that control, and its bitten more than a few Vegas users. You can’t really *see* the difference from 99% and 100% opacity, but this will slow the renders more than anything else.
Steve Mann
MannMade Digital Video
http://www.mmdv.com