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I do not know, sorry ... but I am totally confused by what you did to fix it. Seems opposite. I would generally add number of CPU to increase the speed of VM. Or are you having problems with OSX speed?

Not sure what you are doing, but I have number of VMs at home and work, and none ever needed more than 4Gb of RAM. Eve n that seemed overkill usually... Of course, depends on what you need. But if you have 16Gb total RAM and assign 12GB to Parallels VM, there is very little left for OSX programs.

On my i7 system with 4cores/2threads (=8 fake cores) and 16Gb RAM I assign 3-4 cores and 4Gb to Parallels VM (Widnows 7) and all is running perfectly smoothly.

One thing I do is increase video RAM.

This balances OSX/VM for me reasonably well for me... But as I said, depends on what you do in the VM - what resources it needs.

 

OK, just to follow up. I tested more or less the same hardware as WickIndy, may be slightly newer MBP and with PD10, Windows 7, 4Gb RAM/4 cores for VM, all seems to be working just perfectly fine. If anything PD10 seems to be faster reacting (start/stop,...) then PD9.
One thing I'd suggest anytime speed becomes issue - often I find out that my SSD developed problem and needs to be fixed with Disk Utility. OSX seems to have resistance agains catastrophic failures, but small drive issues cause read/write slow down. Serious. Every time I suspect something, I boot in recovery partition (cmd-r on boot) and run Disk utility. Sometimes it finds problems and helps. Does not hurt...