If WhatsApp cannot read our message, how can the media forwarding happen in an instant?
Leaving aside the WhatsApp aspect fo the question: the effect described can be achieved with hybrid encryption, where the bulk of the data is encrypted under a random secret symmetric key, and the result uploaded once; revealing that data to a recipient is performed by encrypting that symmetric key towards the recipient (e.g. using his/her public key), and thus requires very little data transfer from the sender for each additional recipient.