Is 50% speed decrease useless? Take Allo gen 2, it has 50% resistance. If you MAX boost its speed after a resilient impact it will only just outspeed a 108 Tryko.
In fact even if velociraptor had 50% resistance any slowing move would mean he drops below 100 speed.
Without boosts it’s basically irrelevant. It’s only useful on Dino’s with slowing moves themselves. One could argue Ludia did it intentionally but it’s much more believable that someone didn’t bother doing the maths when choosing resistance values to find an impactful value.
A max speed boosted erlidominus can’t outspeed a smilonemys whose used shielding decelerating strike. He’s 43 speed faster!?! How is that resistance