How about practicing scales and rhythmic exercises at a slower tempo and then gradually increasing it to reach a targeted tempo.
I have an adult student who has been playing for approx. 3 years and can't seem to pick up the tempo. Even with the easier pieces, speed is a problem.
You could have your student play a scale in groups of five of nine notes at a quick tempo. For example, a C major scale could be attacked either hands separately or together from C to G or C to D (the D a ninth above C) at a fast tempo. I was taught this technique in my graduate years.The above exercise can make your student a bit stiff, so you should have some relaxation exercises on hand.