That's the point - you're with me on this. A slow kick that's still a kick doesn't have the same issue as a slow kick that's really not a kick anymore.But... Static balance fires and reacts differently to dynamic balance - it's the brain's ability to adapt responses to suit the situation that means being better at standing on one leg usually means you're better at (say) jumping from post to post.
As for any of this, I can only really speak of what I think I've found beneficial and how.
I can definitely see where you're coming from with the kick scenario - if you just lift your leg and almost let it drift into place (which I must say, I've seen a fair few do).
When I do a slow side kick I force the motion, using the opposing muscles to resist the movement and keep tension throughout - it's quite possible I'm a bit weird in that when I go full speed I just switch off the resistance.
While I don't think it's possible to develop a decent fast kick by exclusively practicing slowly and just expecting it to work, I do think your slow practice would have to outnumber your fast practice by a very significant margin to be detrimental.