When the box gets sluggish check for Spotlight indexing. Easiest way is to hit the magnifying glass upper right in the menu.

If not run Activity monitor, sort by %CPU, see if you have a process that's being a pig.

Not a lot of difference between Parallels and Fusion. They've both been around long enough where sometimes a version of one is better than the other.

Years ago I used Parallels but there were a couple of crappy versions and I switched to Fusion.

Currently Parrallels has the advantage.

VM Benchmarks - Parallels 10 vs. Fusion 7 vs. VirtualBox

I've done the Activity Monitor thing, shows nothing, no hogging. I'll look at Spotlight next time it happens, thanks.