But the fact is that IF the rider is taking too long then the horses are not properly on the aids. If the rider cannot canter/trot/canter with ease, then they should go back a couple of tests to the canter/trot and x/depart at the end of the diagonal.
What does too fast a change (1 or 2 strides) do? Tends to hollow the back and/or tense the horse. What is prep for most transitions? Two hh (alerts) and transition, hence three. What is too slow a change (4 or more)? A horse which is off the aids or out of balance. A little error with ok balance? Fine. A little without balance? Problematic reactions. More strides still? Off or dull to the aids.
Of course there are other elements: the most important is straightness and balance (and bit acceptance/proper tempo as a result). All play into the score. Often one can say poor down, but straight, (too many strides might happen here) but balanced depart, or whatever.