If the fan is connected to say a 4 pin molex then the 2 out of 4 of the pins are ground. But that doesn’t actually help because I think when the fan motors spin they create noise that leaks back up the lines into the PSU. I’ve noticed I like how my system controls better with the case open and just a single fan running on the CPU rather than the thing closed with 4 fans connected.
The PC is still playable with all of them connected but you can just notice slightly heavier cursor movement while it’s lighter with only a single fan. It’s possible some people have a ground problem but I doubt that’s what the problem is for most people and it’s going to be insanely hard to diagnose things like the above.
Then you have other niche things like where I swear I’ve noticed mice that have ferrite chokes on the cords have more linear and slightly smoother movement than ones that cheap out and don’t include them. There’s a lot of tiny issues like this where if you don’t pay attention to them those 10 tiny things might add up to make your system unplayable for competitive games.