I have an Intel Core i5 with only 4GB RAM. Windows 8.1 64-bit runs very well on it as it is. However, I installed Hyper-V and a Windows 7 64-bit virtual machine (allocated 1GB initial RAM but it can use as much as it likes dynamically). With the virtual machine running the computer was thrashing the disk drive constantly! When looking at the Task Manager usually the Disk activity was at 100% or near it. I was lucky when it dropped to 80%! This made the computer very slow to use, especially when I tried to work inside the virtual Windows 7 guest.
I knew the ultimate solution was to increase the physical RAM in the computer but I didn't have any to hand... what else could I try?
I ran the disk Defrag but there was nothing to de-fragment! I checked the paging file but that was fine, it was set to automatic (I know some people say it's best to set it to something manually but in my experience it makes little difference - Windows 8.1 is much more clever than Windows 95!).
I then remembered something I'd looked at many years ago when I first had Vista. I remembered ReadyBoost. When Windows Vista was released it was touted as a way to increase the speed of your computer without adding RAM. The idea is that you plug in a USB flash drive (SSD - Solid State Disks). The computer can read small files from flash memory faster than it can read from the hard disk drive. This can improve performance as Windows is often reading small system files. Back in the days of Vista I had tried it but I'd not found much of a performance boost. But I decided to give it a go, maybe with Windows 8.1 it will make a difference?
I had an old USB 2.0 flash drive, a 'Disk to Go Fusion' 16GB. I plugged it in, formatted it to exFAT. I right clicked on it in File Explorer, clicked ReadyBoost - I dedicated the entire USB flash drive to ReadyBoost (see the screen shot).
I shut down and restarted the computer (this is important, until I did this I saw no improvement).
Once Windows 8.1 was back up and running I tried my Hyper-V Windows 7, I used my PC for a couple of days in this configuration. There was definitely a change in performance!!! When viewing Task Manager the Disk dropped down to 80, 70 and lower. Windows 7 was still not 'fast' but it was usable. It worked!
I added another 4GB RAM and now my PC is running even more smoothly, the Task Manager reports very low disk usage. This is the obvious choice.
Upgrade your RAM! But if you do have a spare USB flash drive, try ReadyBoost. It can speed things up, at least to the point where you can work with the computer.
I still have the USB flash drive plugged in and according to the Windows 8.1 Performance Monitor, it is working... The following is a good blog article I found that explains more about monitoring:
ReadyBoost on Wikipedia
NOTE: What I've described here was not a scientific test of ReadyBoost. I am just sharing my experience just in case it helps someone.