This must be quite an oldish story because I read it as a kid in the 70s. Now I need it badly and can't find it. I read a translation, and translation was named Invincibles or Invincible, but that may possibly not be original title (and it's not S. Lem's Invincible novel)
Earth people are conquering the universe, planet by planet, and from one planet the vanguard sends only a short message, before proceeding to another planet: "Invincible" (as in "these guys are invincible")
Now, the earthlings were very curious about this because they haven't yet met a civilization they couldn't conquer. When they arrived, they found a relatively advanced civilization of smallish humanoids (they had technology, cities, trains were mentioned, etc.). But the inhabitants were so slow that they (and their trains, etc.) looked almost frozen in time. They wouldn't even notice earthlings when they arrived and moved around. You'd have to stand still for like 15-20 minutes for the native to slowly move their eyes in your direction.
I don't remember the plot from that point onward, I only remember that eventually the earthlings gave up the idea of conquering/occupying these guys and left, concluding that they are indeed "invincible", in a sense. Although they could easily wipe out the native civilization (natives would not stand a chance at their speed), apart from that unnecessary destruction, there was nothing else the earthlings could do with these people. They were simply unusable for any kind of exploitation.