To get a idea of the scale we are talking about...
The speed of sound in a low humidity environment is going to be something like 350 to 400 meters per second. I've heard that humans can't distinguish 10msec from instantaneous. That would be somebody standing about 4 meters away from you making a sound.
Probably 100msec is acceptable, I figure. From end to end. It'll sound like your standing a ways away in terms of response, but you can compensate.
But if you tune your system to deliver 30msec performance and Linux will randomly have latency spikes past 150 msec every time a disk is accessed then it's worthless to you.
Realtime is about being able to provide deterministic latency.... Linux can't deliver reliable performance without realtime configurations. You can't really tell what latencies you can rely on because the system is not configured to behave in that way. It will be optimized for batch processing and server workloads and in that you can latencies all over the place as long as that will deliver the best performance in the long run.