Make sure the new motherboard has the same XMP profile selected as before. Nvme drives for games tend to work better without the OS installed on them. This is because as you are playing whatever game it is at the time the OS is also constantly using the drive slowing it down a little for the game you are in. I personally have 2 Nvme drives. One 1tb drive for my OS and games that are not picky about loading and rendering ingame, and a 2TB game drive for all the other games. This is totally unnecessary though, just a nice small benefit of keeping an Nvme drive as a storage drive instead of the main OS drive and game drive at the same time. Something to think about.
The thing is before there was no xmp. It was just the 3600mhz sticks running at 2666mhz on the mobo. IDK what's wrong.
Concerning the nvme thing, I was smart enough to partition it before installing OS. You think it makes a difference to install games on the other partition without OS ?