Several tcc systems have forms with fast and slow parts. Those are dedicated forms, though. Imo, a slow form is meant to be practiced slowly, but a student is free to experiment. Of course, if precision of technique is a goal, slower is better. And, if the technique is faulty, doing it faster won't improve it.
That said, we always used to separate parts of the form out and practice them at full speed. That's how they would be used in sparring. We wouldn't spar as if we were doing a form.
Hey, pick the parts of the form you plan to use against an opponent and link them together to make a fast form sequence and insert it into your slow form randomly.