He may not even know he can.

SPEED is a function of DISTANCE over TIME.

The triangle of Speed, Distance and Time. 
For determining Speed, we're interested in the bottom triangle.


If you go 50 miles (Distance) in 5 hours (Time), your Speed was 10 miles per hour
10 mph = 50 miles divided by 5 hours or 10 mph = 50 miles/5 hours

An equine taking 20 quick short steps may not be as fast as one who takes 15 long steps to cover the same distance although he may look faster because his legs are moving quicker.  Visualize a VSE vs a horse trotting a cross country obstacle course.

Do the 20 quick steps take more or less time to accomplish the same Distance than the 15 long ones?

Which equine will expend more energy or tire faster?

Which is more efficient?

Slowing the tempo of a gait, that is: allowing the horse to take longer, more efficient strides, generally allows a horse to move its body and limbs through a fuller range of motion.

To accomplish this, the muscles need to be supple, not tense.

Encourage your horse to slow his tempo but...now here is the important part....

He may not even know that he can.
Be patient, be kind, allow him the space and time to learn.