I heard someone saying that compressing the Virtual Machine folder in the host system (Windows Server host) would improve performance of that guest VM. I am willing to test this, but maybe lots of people tried it before me and know its true or not..
I remember the first gen of zip disks, if you used drivespace, the device could hold twice as much info and was a LOT faster as well 
Maybe this is also true on some specific virtual machines, for example an email gateway with very little disk activity, in contrast to an exchange/database server.