Did you know that more traffic accidents happen at slower speeds? Why? Distractions! The mind wanders when it thinks it has plenty of time to react. Tennis players fall victim to the same problem when hitting slow balls.
When hitting a faster ball, you automatically focus on the incoming ball and the task of returning it. When “waiting” on a slower ball you might start considering all the options and lose concentration. Given enough time, some players even start doubting their strokes and short arm the shot(tighten up and swing tentatively).
Set up a practice match with a friend that you know hits slower shots and work on your focus and concentration. Remember, practice focusing as much or more on the slow ball so you can avoid “accidents” on the court.