The 18T feels genuinely slow on a mountain bike, and possibly could be meaningful for cyclocross and possibly crit racing on road. Fast engagement doesn't really make much difference for most road riders. I have done a lot of road/CX riding on low engagement hubs and it's never an issue for me.

On a mountain bike, it's a pretty small amount of the time but 18T makes timing pedal strokes when I'm worried about pedal clearance. 36T is fine, higher than that is probably useful for those who regularly ratchet through sections or utilize some trials-like skills.