ok finally i found some 1 with the same issue as the OP, so dont worry your phone is not defected.
I would like to first add a few thoughts in response to the answer above. while it may be correct that mtp is a slow protocol this speed reduction is exclusive to the whatsapp folder transfer only .. any other folder containing equal size transfers in minutes. so there has to be something in the nature of the whatsap folder itself or its structure /specific files thats causing the slowdown.
this brings me to my solution to the problem, I personally use the superbeam app both on the phone and PC .. u do need the pro key for 1$. but this reduces the 24 hour + transfer to like an hour, so this helps greatly. [again will like to point out that this takes much longer that any other folder of the samae size ]