Why do we have to wait for the M61 cannons to spin up?

I really doubt that but ok.

Id love to see any evidence that there is a noticable delay from after you pull the trigger.
Since you doubt me.

vulcan is β€œArmed” before shooting to wind up, or spins as it shoots increasing the frequency .

It will take 0.25 sec to spin up. So it relastic πŸ‘πŸ»