I've seen an explanation somewhere that I likes which is super speed and super reflexes are like tensing two different muscles.

You can make your body move at 10x speed to run across town (even 2-3 speed makes you faster than the fastest humans and 5* is like a car going quickly), then you use your reflexes 3 times faster to make that superspeed feel like only 3 times your natural speed. You can also seperate them in other ways like buying yourself time in an exam by speeding up your thinking without actually increasing your muscle movement.

Your mental and physical speed being like tensing muscles also means you can have them kick in like a withdrawal reflex. As the air pressure changes before an explosion or bullet hits you, you involuntarily crank your limit to 50x speed (or whatever is required) to dodge.

It also has the obvious weakness of being exhaustable. It's worth mentioning that 10x speed is absolutely enough to do most superhuman abilities, and 3x speed makes you basically better than most humans, not just at running but also dodging and punching, and it's absolutely up to the storyteller to decide if such a low speed multiplier exhausts said speedster or if they can maintain that indefinitely like the muscles we use to stand.