back leg

does whether or not you have your back leg collasping during your delivery affect your control, speed, ect.?

Absolutely!

in a positive or negative way?

Very negative! You don’t ever want your back leg to collapse; you need to drive hard off the back leg and get it to near full extension in order to get a long stride. So there is some bend to it but just enough to explosively propel yourself down the mound. You don’t want to “sit” or squat down and then go out - rather you want to glide down and out simultaneously down the hill with a push-off. Some pitchers can get away with a bit more of a back knee bend because they have very well-trained, functionally-strong legs. Most youth pitchers don’t. Even still, you don’t want the energy to go straight down into the rubber - you want it going toward the target as quickly and explosively as possible.