This may be common knowledge for most people out there but I bet there are also many that don't know how to take advantage of this. When I was preparing my demos for PDC I ordered an external hard drive, to store many VMs, and an extremely fast 32Gb flash drive to run my VMs from.
I tried running my complete VM from the flash drive and the performance was awful… I was sooo disappointed. Well, little did I know re: the right setup to make things run snappy (well I did know I just didn’t see the issue at the time or had the naive hope that the flash drive would also have fast writes)... An evident fact that I nevertheless disregarded was that all the reads should come from the flash drive and all the writes needed to go to a hard disk (flash drives have extremely fast reads but lousy writes..still). Thanks to a tip from some folks in our evangelism group I was able to set things properly and what a difference.
The trick was to put the “base” VM in the flash drive and the differencing disk in the hard drive; this in Hyper-V terms means putting your hard drive in the flash drive, configuring your virtual machine settings so the snapshot directory points to a hard drive location (I used a pretty fast external hard drive) and taking an offline snapshot (e.g. when the VM is turned-off) ….voila! You will see a 2x to 3x improvement in VM performance.