When I was teaching my wife how to play shooters I had to study how I was using my left hand on the keyboard.

I noticed that the WASD placement excluded the middle finger. Once you lift it up and keep it hovering, the pinky hitting A, the ring hitting W and S and the index hitting D, it's very easy to dance them.

I find the middle finger much less responsive than other fingers due to tendon connections (it is much stronger though as another user noted for bowling)

I am left handed but I do use the PC like a "normal" person. She is right handed and she played noticeably better once doing this.

The other major thing was to avoid cross talk between hands. Full aiming related things on the right, full movement related things on the left. (grenades, aimed abilities, etc. on mouse keys)

Related note: in paintball, you feather the trigger with the index and middle finger as they are the fastest. The pinky is also fast but the ring is so slow that doesn't work. Middle and ring is slow for the same reason.

TLDR: ring is your muscle finger, other fingers are more agile.