Oooh, this type really burns me, just like the kids who feel obligated to pull ahead of me (and into the pedestrian crossing) at the stop lights, and who then proceed to move at a relative snail's pace after the signal turns green.

In either case, if their pace isn't too too slow, I'll draft close behind for a while with my ease and their discomfort in mind, then pass (closely) when it is convenient.