Arch main, about 700h
People mentioned that problem above, but seems like nobody takes it serious. In 1.5.5 there just won't be any reliable damage source for horses, except enemy cav and archers. The damage of the most common bow (recurve) has been nerfed so that's even less damage for horses. The rider will feel free to ride through the whole teamfight with no fear of catching random throwing axe, and the archer player will not feel safe near his inf team. That rider's freedom of action will cause more bumps, spear hits and impact overall.
My suggestions are:
Decrease horses armour to the point where infantry can deal noticeable damage to heavy horses.
Buff long range spears and pikes against cav, for example add something like 1.2 multiplier to this weapon against horses.
Slightly decrease speed threshold for stopping horses.
Also the rider's leg has a strange hitbox, you can shoot directly in the rider's feet and you will only damage his horse.
What about horse teamhit damage, I have no idea why this was implemented but for me as an archer it feels good)
About bump speed thresholds, decreasing it, even together with decreasing of knockdown time, will buff cav even more, cause experienced cav players will bump almost always. Now the bump is something like pila headshot at the start of a fight, it rarely happens, but it gives you a great advantage, but with consistent like 2s bumps cav impact will be way higher