Even with the fastest cpu's, the amount of alorithms that need to be proccessed with even a few vst's and host programs is a hell of a strain. This cant be helped im afriad. Try and tweak your windows to get the best performance and then work round the problem -

ie - Dont use multiple reverb plug-ins when your using the same setting, put one on a send and use that instead

Bounce down any cpu heavy vsti tracks to audio..

Delete and Vst's that arnt in use

Dont use anything that you dont need to...

Hope this helps