Running Parallels VMs from a NAS
All three of your VMs are stored on your NAS, correct? I am not surprised that they are all slow. You are streaming tons of data over a network and expecting it to work as if it were being used locally.

Virtual machines perform a lot of disk IO. Using network storage for VHD files and other resources that are used by your VMs is not really a great idea if you need responsiveness. A lot of software has been developed to try and solve the issue of streaming VMs over a network. 

You may want to look into the many VDI solutions that are available if you really want to continue using your VMs over a network. Just a thought. 

If this is not a good solution for you, why don't you try keeping a small VHD locally with just the operating system installed on it, and then add the bulk of the storage onto the NAS? That way, the VM would boot/run as quickly as it should (locally), and the majority of your data would reside on the NAS (and be accessible from all VMs and the host OS).



Edit: unless you are connecting your NAS via SATA, in which case I am unsure of a solution to your issue. As soon as I saw "NAS" I thought of you streaming your VM images over a network...