Backspace key on the home row
The Caps Lock key on Colemak acts as an additional Backspace key. This allows to quickly correct errors, without moving the hands from the home position. This also prevents mispositioning errors when bringing the hand back to the home position.
Take off the load off the weaker fingers
The ring and the pinky finger are weaker and slower than the the index and middle finger.
Cutting the distance the fingers have to travel also increases typing speed.
Low same finger ratio
Using the same finger slows down typing.
Easy to learn
Because the layout is easy to learn it means that you'll catch up on your original speed faster.
Touch typing technique
Since your fingers stay most of the time on the home position, the Colemak layout encourages a correct touch typing technique.
Ergonomics are just as important for sustainted speed. You won't be able to type fast (or at all!) if your hands are strained.
Speed is something that is very difficult to assess without long-term controlled studies, and the various parameters affect different people in different ways. Studies that compared QWERTY and Dvorak showed only a marginal increase in speed, however some anecdotes report very significant increases in speed.