I wasn't sure if this should go in Exploits or Bots & Programs because you will need to use XYZ, so I just put it here.
With XYZ running: enter an arena and mount up (this does not work in Dalaran arena obviously)
Click 7 on your number pad once to move yourself into the air
While in the air, right click your mount buff. You will get disconnected
Upon logging back in you will run at 100% speed and can still use all abilities!
The effect is canceled on loading screens, so you'll need to redo it each game
(I can update this thread later with a video, but I'm in Ulduar ATM.)