Has this ever happened to you: You're an up-and-coming necromancer, 100% on his grind, and you mount your noble Friesian or Clydesdale expecting to break out into a vigorous trot, only to find it hobbling along like a gotdamn bob-tail nag? Assuming this is happening to you in Diablo 4, you may just be holding your mouse too close to your filly.
"Horse speed is relevant to cursor distance from character," declares Reddit user FloridaMan156 succinctly in a well-trafficked post on the D4 subreddit, a post that subsequently became a lightning rod for comments with variations of "wait, what the hell, I've been doing it wrong this entire time."
Basically, if you press the "spur" hotkey without having your mouse far enough away from your character, it won't have much effect. Conversely, you can move that sucker far away from your mount and get ready to fly. The source of confusion seems to be that this only applies to mounted movement—hoofing it on foot in Diablo 4 has you going at a constant velocity no matter where you put your mouse.
"Tfw thought the horse was janky but was me who was jank," wrote user edgyallcapsname on the revelation.
Many commenters similarly expressed surprise, but others had some further useful insight on the mechanic. FieserMoep plausibly posits that this relates to Diablo 4's simultaneous development for controllers as well as mouse & keyboard. The mouse position mechanic may be an awkward approximation of the variable travel of an analog stick.
This hardly seems like an ideal solution for horse speed on God's own control scheme, but if you too felt more like you were wrangling a stubborn mule than flying on a noble steed in Diablo 4, maybe this tip will help speed things along.