Take a pcap on your router while you're downloading things and it should be enlightening.

If you don't have a router you can capture pcaps with, setup a box with two nics, bridged, between your ps5 and your router and take pcaps on that.

I'd expect slow downloads to be either a pMTU problem (but I over index on that), the tcp window is too small, or the download server is far away/high latency. Too small window and far away download server combine to make it worse. Sibling's suggestion of adding a squid proxy likely helps because squid is highly likely to have large tcp windows. Other sibling's suggestion of disable ipv6 is likely to help if Sony didn't bother to do happy eyeballs and their ipv6 infrastructure chooses far away mirrors or is otherwise partially broken.

Could also be using a download server with high path loss, that would show up in the pcap as lots of out of order packets.