How fast was the ankylosaurus?

They weren't very fast. they had short horozontal legs, already holding up the weight of their body and armour. At the most they'd move at 6 mph(9 kph) in a fast amble.

Speed wasn't their defense. Their defense was to lower their body and swing that tail at attackers. As such since it didn't need speed it was able to bulk up with all that bony Armour. It's like how a turtle doesn't need to run fast to escape a predator.