Watch the Tremolo Variations online guitar lesson by Muriel Anderson from 50 Right-Hand Techniques You Must Know
There are other variations to do the tremolo. Here is something that may be helpful to develop speed, after and in addition to practicing with dotted patterns (slow/fast).
Start by playing three notes as fast as possible. Then play the first four notes in the pattern as fast as possible. Play the first five notes (the tremolo pattern will end with the thumb). Continue adding one note each time.
Keep track of how many notes you can play in succession before fumbling. Eventually the muscle reaction will develop to the point at which the tremolo can be sustained.