Speed relative to what, exactly? There are no preferred reference frames. A true reactionless thruster could always treat itself as if it's initial speed was 0,

Speed relative to what, exactly? There are no preferred reference frames. A true reactionless thruster could always treat itself as if it's initial speed was 0, and so it took 0.5J to accelerate by 1m/s. Rockets do that too, but they get away with it because they stored the necessary extra energy into the kinetic energy of the fuel. If there was a reactionless thruster that did, in fact, get less efficient at higher speeds, then that would also be world-shattering physics news, because then you could measure it's performance after accelerating in different directions and eventually get a true rest frame that is privileged over the rest of the reference frames out of it.