Wiktionary
subrelativistic
a. (context physics English) Describing a velocity (and associated properties) that is considerably less than the speed of light such that relativistic effects may be ignored
Usage examples of "subrelativistic".
The entire planetary polity can't afford to pay the energy budget of a relativistic starship big enough to hold everyone, even as uploads, and a subrelativistic ship would be too damn vulnerable to the Vile Offspring.