A lot of hitters, especially young, and an experienced ones, will slow their hands down in order to adjust to the timing of a slower pitch. They can still be on time with the pitch by doing this, it’s just not a great way to hit the ball hard.
Ugh. Yes. We stress not "swinging slow" just because the pitch is slow. This happens because they don't understand starting later (thinking about a pitcher who is always slow, and not a pitcher changing speeds). I have no idea how to practice this though. Simply throwing slow front toss doesn't seem to get the job done.