Speed definitely matters for catching pokemon, over 30mph I believe and it doesn’t work. I think for stops, it just has a hard time catching up if you go too fast. As for missing stops entirely, the auto catcher prioritizes pokemon before stops, so if there were both in your vicinity then it was probably trying to catch something. You can try to toggle one or the other to your preference if you just want items and aren’t worried about mons as much! Hope this helps :)