It has to do with scaling; all mounts have to move the same speed regardless of which mount it is or what race is riding it, so the larger ones -- particularly the griffin, which is huge -- have to have slower animations so their longer "stride" doesn't make them faster, particularly when ridden by larger races. They can't speed up the animation without either making the griffin faster for larger races than smaller ones, or making it look like it's essentially on a treadmill and gliding across the ground.