Sure, it’s fiction, and Superman’s powers and limitations are whatever the plot demands.

But if he could move that fast, and he was in a real major city with real people and real problems, then he would be saving people nonstop. Because he could. If he’s faster than light, he could go save everyone without anyone noticing he left the room (setting aside physics, of course). But he’d never be able to stop, and he would never run out of people to save.

And none of it would be supervillains and giant robots or space lasers.

But then, applying any sort of real world rationality to Superman never ends well.