I understand completely. I believe my speed is 175/10, so although the download is great, the upload is rather pitiful. Even with PF, I still get maybe a total upload of anywhere from 500 KiB/s–1 MiB/s according to qBittorrent.
Why does everyone talk about port forwarding when it comes to Mullvad and seeding? When I tried out Mullvad and torrenting last month I was able to seed just fine. Is there something I'm missing?
Not only is UPnP a security risk, it'll work only if a user has an IP that can expose ports. IPv4 addresses are becoming rarer on home networks and CGNAT connections can't expose ports even if one turns on UPnP.
UPnP is widely considered to be a significant security risk, just FYI. Because it allows any random device on your network to poke holes in your firewall whenever they want. You should go disable it on your router.
Plus UPnP doesn’t work in a lot of configurations. CGNAT, for instance, blocks UPnP.