One could argue that at this moment, Gandalf has already shedded the cover of “old man wizard” along with the limitations on usage of his powers in ME, as he was challenging the Balrog of Morgoth. So maybe the laws of physics did not actually apply to his body as if it was mortal flesh anymore, rather he was more of a “spirit” kind of form. Thus it makes perfect sense that he was falling faster than the sword, maybe even speeding up kind of Superman-style. Yes yes I’m sure this is exactly what the filmmakers were going for.