This is going to sound weird, but work out kicking slowly once a week.
Go through your kicks, kicking as high as you can moving as absolutely slow as you can. Maintain perfect balance and technique. Hold the extended kick for 5 seconds, then lower it equally slowly.
Do that till you can't do it anymore with both legs.
That will significantly strengthen the stabilizing muscles involved in kicking. That will give you a great platform from which to build speed. You can't be fast if you don't have technique first.