#1 - June 12, 2016, 3:16 a.m.
This is a problem that has only surfaced since the WoD prepatch - in every prior expansion, all mount speed bonuses were correctly being calculated from your mounted speed. I believe this may have actually been an error on part of the devs.
Now before someone inevitably jumps in with "But this change is intentional! Mount speed bonuses are just additive now, not multiplicative!", allow me to explain why these terms might not mean what you think they mean:
Check out this post on movement speed from the 6.0 dev blog:
Notice how at no point in this segment is a mount speed buff mentioned?
It seems clear from the dev blog that this change is purely talking about stacking multiple run speed bonuses and how they interact with each other. This made sense with the upcoming tertiary stat system in WoD and the ability to potentially roll speed on every single piece of gear - stack enough of these together along with bonuses like Unholy Presence, Death's Advance, the movement speed cloak enchant and Body & Soul, and one can quickly see how this would get out of hand if they were multiplying off each other. However:
The additive/multiplicative dilemma isn't relevant to mount speed, because there aren't enough mount speed bonuses in the game for it to be an issue. Also, the strongest mount speed bonuses (Heart of the Crusader/On a Pale Horse) have never stacked with other mount speed bonuses anyway, so it's a non issue.
There are two separate categories of speed bonus in this game - movement speed and mounted speed. If you don't believe me, try equipping a piece of gear which has a tertiary speed bonus, and mount up. The bonus won't make you any faster on your mount.
I'm proposing that mount speed bonuses should be changed back to calculate from the base of the speed you're moving once mounted. They're currently giving us such a small increase because they have incorrectly been lumped in with movement speed calculations for on-foot running speed.
Please fix this for Legion. Mount speed bonuses are currently very underwhelming as they are increasing mounted speed by a percentage of our running speed when they should be increasing it by a percentage of our mounted speed - they did in every previous expansion. As someone who actually picked a Paladin in BC & WotLK as my first alt because of how cool I thought Crusader Aura was, I'm a bit of a mount speed junkie and this change being reverted would mean a lot to me.