When you're riding your mountain bike you can’t really honk it in a big gear to get going faster. You need to stick to relatively easy gears so that you're always in a sensible gear to take on any obstacles that could be lurking just round the trail. This way you can keep a good momentum and take less energy out of your legs.
Stopping and starting all the time in a hard gear will soon take its toll. So, try doing the following exercises to make you pedal faster using the same gears.
Smooth not stomping
When you’re next out riding focus on where the power is in the pedal stroke. If you're fairly new to the sport, you're probably stomping hard on the down stroke. Imagine that each pedal stroke is a full circle and try to push and pull the pedal round as if you're keeping the pedal to the outside of the circle. Pedal in circles for two minutes at a time to start and build it up to 10 minutes, by which time you'll be getting far more efﬁcient and be going faster without needing extra power.
hope this helps guys. let me know how you get on