Tried this on my Tyreals Charger. Slowed me down too.
Wow, I just tested Armored Bloodwing, Winged Guardian, Imperial Quilen, and Tyrael's Charger while Sanctified Wrath was active (same buff, too) and all of them were flying at half speed. Then I ended with a Snowy Gryphon (poverty pigeon) and it was flying at the customary 492% speed.
So this is a store mount bug? Odd. I think I should file an actual bug report then, since it's not merely my favorite mount being quirky with my favorite class. It's an actual thing. :-(