To put it another way, I have never asked a pitcher to throw slower because her ball was too hittable at a faster speed.
I have. And it worked. My pitcher was worried that the better hitters on the team were timing her up a couple times through the order. She was trying to figure out how to throw it harder. I told her if she couldn't, and just magically throwing harder deep in a game is not likely), then throw it slower. She struck out the 3-4-5 hitters the next inning by slowing it down.
I wouldn't tell her to throw at that slow speed all the time or for a whole game or anything, but for a few batters, it worked very well.