The spell "swift feet" was a spell that was similar to the lightning cage of the lightning element in the sense that it would only disappear when the mage wanted it to.
While the lightning cage allowed a lightning mage to lock a target in place and was still able to cast other spells, swift feet would cover the Mage's feet in some sort of boots which were made of mana and would increase the movement speed of that mage greatly.
The girl who was fighting Laura at the moment was proof of the spell's usefulness. And just like Lightning Cage, the Mana Boots would not disappear as long as the girl still had mana to sustain it, and she could still use other spells without the two interrupting each other.