This also happens to me on the PS4, but only after playing for an extended period of time... or if I do anything that tends to freeze the screen/has lots of particle effects (i.e. WOD dragon run). It's usually resolved for me by shutting the system down for a minute and restarting.
It's slotted in spell mastery! "Spell Mastery: You and allies near you receive Combat Advantage and a Run Speed buff based on how much time was stolen, and your stacks of Arcane Mastery. "