As you take to the skies, either riding Grimtooth's bats or by your own means, you somehow manage to catch up with Rothul Shuh. As you get closer, you notice the reason: his griffon mount is partially covered in some mysterious, crackling dark energy, probably a leftover from his battle against the tomb legion flying undeads. This energy is clearly sapping its vitality, slowing down its movements.
However, said dark energy is clearly dissipating. It will not be long before Rothul Shuh's mount regains its full speed, and easily lets you behind once more.
For now, Rothul Shuh doesn't seem to have noticed you flying behind him under the blanket of the night, too worried observing the burning city below, as the Tomb Legion steadily pushes the Order of Truth defenders back.
Rothul Shuh now at 130 feet away from you, performing a "flying run" of 160 feet each turn, meaning his mount's base flying speed is now just 40 feet.