On machines where you start Steam manually in order to play a game, do you keep Steam running in the background after play session is over? Meaning if you no longer play any game for the moment, and just keep it running to be ready. Or do you close Steam, after the play session?
Frame times are notoriously fickle. All it takes are two tasks that are intensive for 1/60th of a second to be run at once to get a spike. The key is how noticeable are the spikes?
Most of the things you removed to strip down windows probably have much more of an impact than steam, and the 500mb of ram it uses does.