The thing that jumps out at me is that you are using VMs. While the "headline" figures for VMs can look very good they can suffer some severe performance bottle-necks. In particular the disk subsystems can be surprisingly slow - I've seen SAN performance an order of magnitude slower than local disks!!
How is your MUC configured. Are you using memcached or just relying on the default disk based caching? Having you run the performance tests in the MUC?
You are running on Windows, which isn't the greatest idea but should work. However, what monitoring are you doing? Are you checking what the machine is doing while it is serving Moodle pages? Are you definitely not running out of resource somewhere? No point assuming, you need to check. I can't give you specific advise as I have always managed to avoid Windows.