it's not so much the running processes, as it is the unpredictable nature of scheduled tasks which do things that just suck performance away from games. those things often run with very odd priorities and very odd schedules or triggers. Also, when games are run and windows doesn't really get to see the mouse move normally because the game has captured it and controls it, windows sees this as "idle" time in some ways, and will handle background activities via the task scheduler. in any use pattern that is not gaming, this is the correct behavior.

a windows OS mode that is single user, networked, and meant for limited use (so background housekeeping windows tasks run after you exit or reboot out of this mode) would be pretty ideal, not just for raw performance (which would be measurable, but not the main goal) but to avoid apparently random decreases in performance due to Windows deciding to do something, or suddenly needing to deal with an authentication request dealing with mapped drives, or something.

Basically, Windows needs a "Game Console Mode".