Zhaneel is heralded as the fastest flyer of her time, and the newest of a subspecies of gryphons based on the slimmer gyrfalcon (rather than a broadwinged eagle or hawk). Taking the species and her larger size into account, I tend to go with the species numbers here:

Typical horizontal flight: between 65-80mph
Fastest horizontal flight: approx 100mph
Stooping (diving): 200+mph in a free dive stoop

All these can be tweaked if you need. I just went with species numbers for an approximate speed.