Cheap Shot is either bugged or Riot changed what "Movement Impaired" means.
Hit Xerath stun, Arcane Meteor hits, Get the bonus damage from Cheap Shot. Hit Xerath slow, Arcane Meteor hits, don't get damage from Cheap Shot. Didn't really use this rune on other people so I don't know if it's all slows or just Xeraths.
Edit:
You know, now that I think about it, maybe it's not. Maybe even though it's a 2.5 seconds slow, the Arcane Comet just isn't hitting in time.