Earlier quoted context omitted.
A true real time system is likely over kill. General time-sharing systems generally are accuracy to +/- 4ms. Which is acceptable for most applications, as 60fps is ~16ms per frame. If you (ab)use Linux properly you can get most your threads to only halt on blocking for IO. So they'll get scheduled practically (not always) once that IO state is satisfied. This requires heavy multi-threading not horrible, just isn't am…
I would guess the PID loop benefits greatly from real-time execution. So much so that really the base control system should be on a dedicated controller with AP functionality as a secondary system driving the inputs to the PID controller.
It really depends what you are controlling. With a PID loop generally more accurate timing is the battle cry for moving them off board. As implementing a PID in a time-shared OS is rough as you don't get timing guarantees. So your Delta-T is always off. Generally by a margin of 1% which is large enough to throw off a correctly tuned system.
Faster updates are nice, but not always needed.