I work with adults mostly and when we work on speech rate for Dysarthria I like to use the CSL (Computerized Speech Lab). It has a feature on it where I can record the pt's speech and then replay it at a faster rate, so they can try and match it. Of course, it depends on "why" their speech rate is slow....coordination, motor planning, etc. I have also used the feature on the CSL of the metronome but I find that elicits staccato/robotic sounding speech. I bet you could find an app or something that was similar to the recording feature that allows you to modify the speed the recording and play it back for the pt to imitate. Hope this helps!