I noticed a while back that if running a game and a bandwidth-intensive task at the same time in Win10, the game's framerate would vary a lot depending on which window was focused, despite there being plenty of CPU cores and threads to go around. I don't remember details, but it might be worth trying to recreate this situation.
Windows does two things to focused windows:
1. Give a focused window priority boost. This means that a a focused window will get more CPU time than others (not relevant in the case of "plenty of CPU cores to go around", because all tasks will execute as long as cores are available)
2. Give the focused window 6-epocs per task switch, instead of 2-epocs per task switch. Focused windows have fewer context switches. Normal tasks switch every 30ms, while focused windows only switch every 120ms. This ought to improve framerate slightly, especially now that context-switches are way heavier (thank you Spectre, which still affects AMD)
What I'm doing now is running four processes (one thread each) of a random throughput test. Each process does pipelined random reads from a 1GB allocation. In Linux, each process spends most of its time at 46-50 Mops/s regardless of how long the run is, as expected. In Win10, one process always wins to the tune of being 50% faster than the other three, even if I let the whole thing run for 5 minutes. It is usually, but not always, the one that launches first (there's no more than a half second between launches). Which window is focused doesn't seem to have anything to do with it.
Does anyone have any idea what I might be looking at here?
Unfortunately not. I was just grabbing the easy question you posted, not the hard one. Lol. I've noticed that "Memory-Testers" launched simultaneously seem to perform greatly differently on my Threadripper box, but I never thought about questioning why it does that. So I think I've personally seen some evidence for the behavior you're describing.
I just don't know why Windows would be doing that.