This constellation should have lower latency over long distance than is even theoretically possible using terrestrial fiber, because the speed of light in space is substantially better than the speed of light in fiber. [0]

Even over short distance the latency (which the "Legal Narrative" pdf quotes as 15ms) is negligible for almost all applications.

[0] paper http://nrg.cs.ucl.ac.uk/mjh/starlink-draft.pdf / video https://www.youtube.com/watch?v=AdKNCBrkZQ4

> than is even theoretically possible using terrestrial fiber,

If we're talking about theoretical limitations then hollow core waveguide fibers should get you close to vacuum speed.