My experience with every version of windows is that after you first install it, it proceeds to churn, churn, churn the disk running diagnostics, updating itself, running antimailware etc. etc. etc. The best fix is just to plan to do your installation at the end of the day so the damned machine can just churn all night. Okay, I am exaggerating but Windows really does a lot of tedious churning. A couple of days after a new Windows installation it seems to calm down. Then, every month around the second Tuesday ("Patch Tuesday" they call it) you get another dose for awhile. I agree with giving the VM as little RAM as possible--especially if you have SSD. I run a Windows 8.1 installation in 3GB of RAM.
I see complaints all over the Parallels site about Windows hogging resources and working slowly, blaming it on Parallels. Nope. It's almost always Windows. Exceptions would be the odd bug here and there which the Parallels team seem to be relatively good at fixing. I used to ping pong between Fusion and Parallels. I stopped it. They both face the same problems (aka: the beast that is Windows) and they both work very well. I just happen to like Parallels a little bit more.
Last edited: Nov 15, 2013