If they are traditional "risen dead" zombies (not deities), they could be fast at first (but no faster than they were when alive) then they should slow down as their muscle and nerve tissue rots.
+1. I've never understood (apart from exiting movie making) why zombies would have enhanced strength, unless lack of nerves gives full potential, but this would rapidly diminish as entropy takes over. Sorry, I know it's boring but I like to think of the 'why' of things. Of course given how much aliens are already responsible for they would find a way around this.
Unicorns are smarta$$es. Don't trust 'em. Zombies, OTOH, are at face(eating) value. You know where they stand/shuffle.I'm not a Fan of Unicorns...............
I think the Zombies of our future will surprise a lot of people. I think they will be kind and nurturing to humans....and then eat them.