Shows a small number that shows your character's movement speed. I don't know why they took this out of the default UI.
If you're standing still, it shows 0%. You can click and drag this frame anywhere you want and it should stay there. To see the number change, move your character!
Slash commands are:
/movespeed reset
This moves the frame to the middle of the screen (0,0)
/movespeed bg
This adds a backdrop to the frame so you can see if the frame is overlapping something
/movespeed bgoff
This removes the frame so it looks nicer.