There are different versions and areas which people refer to ability delay, they are likely different issues but may be related. Like the abilities not firing comes into this category also the abilities using the GCD and not doing damage, animations starts then stops (gets interrupted by something when its meant to be instant), and the ability delay where abilities are delayed by longer than the cast time or animation.
People have just generalized ability delay to the mount thing. The real name given to this issue should have been faulty ability response.