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.

If it's anything like the PS4, you might be able to fix this by running a HTTP proxy using Squid and configuring that on the PlayStation. There's something really funky about Sony's networking on them, and configuring a squid proxy for my PS4 vastly sped up my downloads.

I tried to find an article covering this but I've had no luck, so I'm just going off memory here, but I'm pretty certain that's all that was required.

My guess would be that this has more to do with disk speed if your download speeds are decent. Especially if it’s wired to the internet rather than WiFi. It’s always easy to spot the console user in cross-platform lobbies because of the loading times.

Playstation's networking capabilities are an absolute mess.

In anticipation for GTA6 I actually bought a PS5 Pro 2 weeks ago. I had to create a separate IPv4-only network because it would not complete the user-sign-up on the device on my IPv4/IPv6 network.

Absolutely insane.