Do monsters also slow down when in combat?
Technically I don’t think so. Practically most of them actually speed up as they start to chase you instead of wandering around aimlessly.
As far as I can tell, mobs only have one speed. With the exception of some mobs that have speed buffs (e.g., Risen Thrall) mob speed seems consistent with character in-combat speed.
The numbers on the Movement Speed page have been changed a few times over the years, so it’s hard to know what true speeds to measure NPCs against.
My guess is that balancing would have been easiest by standardizing all movement speeds, and then giving players the out-of-combat, forward-movement speed buff. Other NPCs can then be given their own speed buffs without “fun” coding conflict (I don’t want to see some of those risen move any faster). Of course, I have no idea what layers of code have been made during development.