really? I just can speak out of own experience as a target, some time a HA charged me at full speed and his tatar arrow took 60 % of my health, and that with 21 str, 4 IF, fully lordly heavy armor. And no, it was not a headshot.
Just saying. Bad luck?!
Aye bad luck, you could have been running towards him, which adds to the damage, but not his speed. Also WM items help. Although if you say 60%, more than likely it was a headshot which bugged on the graphics side.
I tested, realized that speed bonus does not apply, so asked the devs about and they said that it is so for balancing purposes. (i also think that someone said it was hardcoded, not sure about it though)
The best way to see this is to make a skip the fun HA, follow someone on the same horse at top speed. He will get speed bonus on his defence even though you are traveling at the same speed. Do the same standing still, he will die much faster. This ofcourse is completly inbalanced but wont be changed.
FYI - Im a HA with champ courser and MW horn bow that shoots at speed, so id notice if the bonus was applied