Created a world back in an earlier snapshot. I've created a stopwatch and timed the horses on a racetrack. They varied in speeds from 7.4 up to 12.2 seconds.
In the latest snapshot, they all perform the exact same speed. All now 10.5.
But new horses spawned into the world vary in speeds again.
Thanks! Having a great time with all the new features!!
This should be expected from snapshot to snapshot as Mojang tweaks the horse code. As long as the horses spawned in the current snapshot vary properly, it should be fine.