Diablo 4 makes your horse faster in Season 16, but its top speed barely changes
Your space bar can relax a little. Your top speed, not so much.

The 3.3.0 PTR patch notes revealed some interesting details for Diablo 4's upcoming Season 16, and possibly one of the best changes is something for anyone spending a lot of time on horseback, as mounts will be faster by default.
The downside of this change, however, is that Spur now gives you less of a boost, so your horse's top speed only goes up slightly.
Faster Horse, Weaker Spur
Here is the official line from Blizzard's PTR notes:
Game Feature Updates
Gameplay
Effects that deal damage over time in a specific area, such as Upheaval's Trailblazing Variant, now have their duration increased by bonuses to Damage Over Time Duration, such as those granted by Shattered Vow.
Mount speed has been adjusted to be faster baseline, but benefits less for spurring, such that the maximum speed is only slightly increased.
This basically means your everyday riding speed should feel quicker, and you will not need to rely on Spur as much. If you are already galloping everywhere by using Spur repeatedly, though... you will barely feel a difference.
Blizzard has not shared any numbers, so we do not know yet how much faster the baseline really is.
Blizzard Has Done This Before
This is not the first time your horse got a sneaky little speed boost. Back in Patch 1.2.0, base mount speed went up by 14%, but top speed stayed exactly the same. Season 16 seems to follow the same idea, just with a small bump at the top this time.
Try It on the Season 16 PTR
You can test the new mount speed yourself in a few days, as the 3.3.0 PTR is going to be available for players from October 27 to November 3 (on PC only). Everything here is subject to change before Season 16 goes live, and Blizzard has not announced a launch date yet. And if you are riding to Helltides anyway, those got some love too.
In any case, this is great news! Your horse is getting faster. Just not racehorse faster.
