My

50 km/h is too much in my opinion for charging cavalry (except in dramatic fiction of course)
A similar speed was impossible for a horse with muscolar, heavily armed warrior on his back in napoleonic time... so it was equally impossible in ancient era, when husbandry was probably less developed.

And anyway, a similar speed would be very dangerous even for the attacking horseman, and would disrupt any resemblance of formation: contrarily to popular belief, a tight and ordered formation was the most important factor in the chivalric mounted shock tactic (and probably in the mounted shock tactic of every horsemen in whatever time...)

So I think a top speed of 30 km/h is more realistic for ancient cavalry (and medieval as well)

Cheers