Think it's to do with it being treated like a zip file. Downloads a smaller version of the patch and then copies/extracts" all the information in it before replacing the original smaller file with the actual patch. Suppose to save on bandwidth and also make it quicker for people with less good internet to download on PS4 since it's a smaller file and you don't need to be online for the copying stage so internet speed is irrelevant, at that point it's down to your PS4 processing speed which as someone said, isn't ideal these days. PS4 limitations