So i was tracking the Qantas Flight from Sydney to DFW it said the plane was descending 3000 fpm
Maybe it was? Controllers are allowed to say things like "QFA7, descend and maintain 5,000, please expedite. Apparently planes would descend at twice the speed but I don’t know if that part is true.
They can descend much faster than this if need be (i.e. in emergencies). If there was loss of cabin pressure for example, they’d need to get to a lower flight level asap.
Especially in busy airports. Center/Approach will clear to x feet. How you get there is up to you. Key is to keep is smooth as possible.
FedEx and ups as I’ve said before drop much much faster.
It could also be that the information is inaccurate, the app could also be estimating, I don’t believe it updates every second. (Just a theory)