The funny part of any teacher-student practice(s), if we continue it must at some point be without "them": Music, Tai-Chi, Pottery, something very dynamic...all the same IMHV.
Speed and balance are the same type of creatures. We must have both. Too fast and both the intent and any pretext of self-control simply disappears. Too slow and the effort becomes excessive initially at least..
Particular structure, anatomical principles must be followed. Remove them.... (cringing) and its only an exotic dance (sic. literally) and badly done. One of the foundational, fundamental intents behind the slow motion idea I believe. Done too fast you cannot focus or pin down most details -gentle shrug-
Heel kick, yikes.....means raising the toes up toward my nose, as I understand the concept at least. regardless of height a thrust/push quality of the heel and parts attached to it.
Ever borrowed a copy of the "Idiot's Guide to Tai Chi?" Cant remember if that was the book Im thinking of, but believe it addressed pain.discomfort particularly in the knees, as I recall anyway. See if I can recall for sure,
Neijia, "internal practice" isn't supposed to be painful.... "discomfort". ohhhh yeah. But outright pain not that i've heard of. SOrry to hear you're in pain.
You could contact one of the more senior Paul Lam teachers and ask if/how to modify their form(s) to produce manageable levels because of knee pains..