Why does a fish swim faster than jellyfish?

Not all fish swim faster than jellyfish. Seahorses are limited in their swimming capabilities and male angler fish attach themselves to a female and become a parasitic sex organ, never to swim again.

Those fish that can outswim a jellyfish can do so because they have a more efficient propulsion strategy, stronger musculature, a more sensitive sensory system and are more streamlined than a jellyfish.