Ah, the joys of trusting a LLM for story IDs...

This one actually exists (a rarity in my experience), and the plot largely matches the description up to the last sentence (though the speed isn't exactly reduced; it's made a constant)

>Humans discover they can modify the speed of light but find they can only make it slower.

That doesn't happen in the story. It ends with the aliens disabling the device (the drive of a derelict starship) generating the field that caused experiments to suggest the speed of light is constant, and then leaving, destroying "the whole basis of their physics, but that is just a fantasy anyways."

Thanks! This wasn't it -- the story I read was a short short story, maybe only a page or two. It was basically:

   1. Humans despaired that the stars were impossibly far away, due to lightspeed limitations
   2. They realized it is possible to <do something> to get around that
   3. They worked hard to try it out
   4. Surprise! They made it worse/it was worse.
   5. The end

I don't remember any characterization, or narrative, beyond the above.