The slow hard drive is what's "slowing you down".
You need to boot and run from an SSD.
BUT -- you DON'T have to pry open the iMac if you don't want to.
You could buy an EXTERNAL USB3 SSD such as the Samsung t5 or this:
http://www.amazon.com/exec/obidos/ASIN/B00ZTRY532?tag=delt-20
Then, set it up to be the boot drive, and put Parallels on it as well.
It will run with about 85% of the speed you would get if the drive was internally installed.
BUT -- you won't risk breaking the iMac by prying it open. Are you SURE you could do this without problems?
Adding RAM might help in this case.
But again -- an SSD will make THE MOST difference.
If you "go the external route", you can be up-and-running in perhaps an hour.
As long as it takes to get everything set up on the external drive.
One other thing:
If you're doing a lot of stuff in Windows, perhaps the best way to do it is on an actual Windows computer.
Set both the iMac -and- your PC up in the same room, get a chair with wheels on it, and work that way!