There are several Mac related posts on this, but nothing specifically Windows based.
PM5 was fast. The first version of PM6 that I installed was significantly slower, but I could live with it; the slowness was mostly when loading the application. The latest version of PM6 (PM6SetupR3023,msi) is SIGNIFICANTLY slower still. It takes forever to load, and when using the arrow keys to navigate from picture to picture, it often stalls so long that you have to hit the arrow key again to advance to the next picture.
Windows 10 Home, 64-bit
Intel Core i7-4770 3.40GHz
RAM 12GB
All other apps have not shown any sudden decrease in speed, so I'm pretty sure the PM6 slowness not system related or due to some Windows 10 update that got stealthily added.
I've reinstalled PM5 and it's what I've opted to use for the moment.
Editorial note: I bought PM because culling photos in Lightroom was unbearable. The performance I am getting in the latest version of PM6 is approaching the slowness of LR, which kind of defeats the purpose of getting PM5.