You might want to upgrade your ram and that could solve most of your problems.
Well, my concern is - do applications run faster under Vista than XP, just comparing the OS and nothing else? For fairness of comparison I suppose should really have both machines with identical spec. - otherwise, more modern Vista PCs (which require greater resoures) might be the performance increase rather than the OS itself facilitating it.
Or, if I rephrase the question:
"My app performance sucks on Vista, if I downgrade to XP will I see a significant performance increase or decrease wrt my application?"