I had a problem with disk space on my Mac, so I figured I should move my Parallels folder to en external SSD to make room. I bought a Samsung T3 and moved all the Parallels files over to it. When I restarted the Windows installation it was really slow, I mean, like 100 times slower to start up. After some research I found out that Parallels work very badly with ExFat which is the filesystem Samsung equip the T3 as standard.
So I moved the files back to the internal SSD, reformatted the T3 to Mac OS Extended (Journaled), and moved the files to the T3 again. And, voila, the speed was back again.