Is there a speed difference between the mounts?
Solution 1:
Having completed the game, and from doing some light testing with each mount, I can safely say that no, there is no speed difference between mounts.
I tested by taking each mount in a straight line from the campfire at Sunfall to the campfire slightly South-East of Sunfall. Times were relatively the same, save for small nuances when the mounts slowed themselves and I needed to press X again to get them to gallop.