If you put everyone's travel speed at the same rate as their reaction speed, that means in the time it took Tony to retrieve a tossed Shulkie a huuuuge amount of relative time would pass for all the combatants, meaning Superman might mop up anyone left for example. But otherwise you'd be looking at this huge bull rush thing. And an impact at near light speed is going to /hurt./ That is the sort of thing that Superman uses to take out moons.

You know, when I put forth that query, I expected other folks (especially Pen) to have an answer for it. I was so sure other people would have an answer for it that I was almost embarrassed to write such a dumb question. But it does indeed seem to be a toughie.