Hard to say why it's running slow. The only surefire way I know to make PHP run fast is to install it on a Linux box 😉
Barring that, what earlier versions were you running, if any, and if so, does going back to them solve the performance problem?
I'd guess that some service on the Windows boxes might be getting in the way, or something might be eating CPU cycles. What does the task monitor show in terms of load?