I’d like to add some light to this, to me it seems to be from a Windows update, I am not sure which one, but since one of those Windows updates, it doesn’t matter which game it is there’s sometimes a bottle neck in fps. It does seem a restart does fix the issue to a degree.
I am currently digging around for a fix like the “Empty Standby Memory” issue. I am currently using that method and it is stable, if you aren’t using this, I suggest to follow that guide, trust me. It works on Desktop / Laptop’s.
I also read that Graphics Drivers don’t resolve this issue, others have also posted that it doesn’t matter if Windows 10 gaming is enabled or disabled. I’ll still continue to dig around because I am getting the same thing in Guild Wars 2, and I have tested with World of Warcraft Retail and Classic all of which after some time of game play the fps reduction just spoils the game.