Which version of Windows works best with Parallels 10.4 and El Capitan 10.11.4

HELP...PLEASE?
I purchased an iMac after using Parallels on my Macbook for a year+ with no problems. Now, when I run Parallels, everything goes extremely slow (ex: applications take 30+ seconds to load, there is a couple second lag between pressing letters on the keyboard and letters showing up on the screed, etc.). Anyone have any advice on how/what to fix, or if I need different versions of anything?

Here are my versions:

• OSX El Capitan, Version 10.11.4
• Parallels Desktop 10 for Mac, Version 10.4.0 (29337)
• Windows 7 Professional, Service Pack 1, 64-bit Operating System