One of the best ways to get faster on your mtb is to spend more time training on your road bike. Most good mtb'ers spend about 80% of their training time on the road because mtb riding is not a consistent aerobic effort (you go hard, you coast, you go hard, you coast, you stop, you go hard, etc). To go faster, you first need to build a large aerobic base and the best way to do that is to log long, consistent road miles at low- to mid-range intensity. After you've built an aerobic base, you can work on specific high-intensity workouts to improve your high-end fitness.