The surest way to play something fast is to play it slowly and cleanly, then speed up in small steps. The speed trainer does the stepping for you: it starts at your tempo and adds a few BPM every few bars until it reaches the target.
Above, it starts at 60 BPM and adds 4 BPM every 4 bars up to 100 BPM. Change all three numbers in Advanced, turn on “Start again when reached” to loop the climb, or set the target below the start to slow down instead.