Yeah, the whole thing gets messy in an interesting way.

- if the rod travels across 7 light seconds in a round, the only way to avoid breaking relativity is if the 6 seconds the round takes are measured in the frame of reference of the rod

- that would mean that from the frame of reference of the rest of the people/monsters/edgy antiheroes/misunderstood blob creatures, the rod’s “turn” took 7 seconds.

All characters in the D&D universe are accustomed to a reality where each round takes 6 seconds, and everyone - in synchrony - is able to perform an integer number of tasks that fit within that timespan. Rounds begin and end simultaneously for everyone involved in combat. How disturbing would it be for such beings to see those laws broken?

Perhaps time is quantized and to the outside observer, the action takes 12 seconds?

Given how grid-based combat works, I'm not even sure that D&D exists in a Euclidean-adjacent space time.

I demand at least a semblance of pythagorean distances for this reason; (N+M/2 is close enough for the distances involved in combat). The 5e default of diagonal moves being equal to grid-aligned moves is significantly more painful to my brain than dividing by two is.

Nothing in any of the D&D universes suggests that it follows the same physical laws as ours, and many, many things say otherwise.