Well, VM package contains your "hard drives" - which is your "C:, D:, ... drive" with system, applications, and possibly (bad idea) your data. Note, that the VM drives these are just files in OSX, which look to VM as drives...
If you need to start system, load applications, read data, ... , these need to be read or written. And speed of reading/writing will impact the speed with which the Windows (VM system) itself will be able to respond. Same as in regular computer - if you have slow drive, it is slow - if you replace it with fast drive (SSD), even slow computer becomes _MUCH_ faster as read/write of data becomes much faster. Actually, old Pentium 4 with enough RAM and SSD is quite usable computer. I tried!
My VM "hard drive" has ~25Gb, my VM memory allocation is ~3Gb. System in my VM has no chance to cache everything, so it will be reading/writing to my VM "drives" as needed. Faster read/write to these VM drives faster my VM will work.
In the mean time, I have got one "leftover" older SSD drive and put it in USB3 enclosure. I measured speeds read/write around 220 MB/sec. I moved my VMS there - really fast enough. Tested.
NOTE: I strongly suggest storing data (as documents, images, music, etc.) on Mac disk where it is backed up by Time Machine or other means, and leave only applications and system in the VM drive. I have seen way too many people here crying, that their VM crashed and now they need to recover some data. System and applications are usually easy to reinstall, but if you loose data, you are toast. You do have backup, do you?
