I agree that some NPCs are too fast for their own good - but I'm not sure
slowing them down is always the right solution.
Hovering over a NPC could bring up a sort of pie menu that would stay at the
cursor position, while the NPC would continue wandering around. The pie menu
could include the ability to activate the chosen program on this NPC or start a
conversation.
Friendly NPCs could also stop moving and turn to stare at Tux when hovered over
(as if Tux called to them), though in some ways that might be worse than slowing
them down, and can get NPCs stuck under the cursor if the player is idling...