Here's an example, I have a late 2012 Mac Mini dual core i5 and it can easily run 3 VMs at once, however I do have an SSD. When I had the hard drive I found the VMs to run really slow, but the Mac side was still fine.
Yep, works very well. I've been running Win7 x64 on a Parallels VM for a couple years now and haven't had a whole lot of issues. On average the VM is pretty snappy with most applications I've tried.