A new version of Windows 10 was leaked, and it apparently contains a file called “gamemode.dll”. Some people are speculating that this could be API hooks for applications to request higher priority with CPU and GPU resources, increasing performance. Microsoft gave a "no comment" to PC Gamer about the issue, but Windows Central cites anonymous sources claiming that this is the case, and it is either related or analogous to how the Xbox One multitasks.
While I believe the DLL exists, of course, I’m skeptical about its function — but I would bet that it’s true (given how sure everyone seems to be). Also, I’ve seen GPGPU compute times fluctuate wildly by leaving other windows open, so there is probably some overhead that the OS can reduce. It just seems weird that this has come out of nowhere.
We’ll probably find out soon, because the Creators Update (Windows 10 1703) is coming out in a few months. Whatever this DLL is, it seems targeted at that feature update.
You can also check out other features of the leaked 14997 build, listed by Paul Thurrott.