The way to play quickly is to practise slowly. There is no better example of proving this advice to be true than by working at Double Tonguing.
Start slowly, making sure your doubles are evenly played using tu ku tu ku.
Your practice tempo can be increased by increments of 10 beats per minute from 50 to 190 bpm.
You can choose your Start and End Bars.
It will still be hard work...... but etos practice tools will keep you company during your practice sessions.
You will be encouraged by the results of time well spent.