There is a counter intuitive way to build speed and make your playing more secure. It is to slow down. When you’re learning a new piece of music, you need to insure you aren’t learning mistakes. If you make a mistake, you’re teaching yourself to play the piece with that mistake. Whenever you make a mistake, the first and most likely cause is that you were playing too fast.
Slow down and make sure you’re playing everything correctly. When you can play what you’re working on correctly, then you can increase the speed. It should be obvious, but it isn’t – slowing down will decrease the amount of time it will take you to learn. Slowing down will also make your playing more secure and as a result you will be able to build speed faster than you could otherwise.