Seeding torrents will be next to impossible without it, especially for private trackers. It's pretty vital for torrenting. Anything else, doesn't matter.

I used Mullvad for a year and love the service, but they explicitly don't allow port forwarding, so I recently switched to Proton. Their CEO's comments last year still feel disgusting and I trust them as a company far less than I trust Mullvad as a result, but the service they offer is stellar regardless.

From what I understand, Mullvad is the top choice for a privacy-focused daily driver VPN, while Proton is the best choice for torrenting while still functioning just fine for daily use.

The only realistic answer is “it depends”. Torrents require at least one side to have an open port. If your port isn’t open, you’ll only be able to connect to people who have opened theirs. If everyone had open ports, you wouldn’t need to worry about it. But the reality is that many people don’t bother with opening a port, so your connections will be limited.

The issue will be much more apparent on smaller/less popular torrents. With popular torrents and hundreds of seeders, chances are good that some people will have an open port. But your speeds may be limited, because you can only connect to a select few. Even worse, torrents may stall entirely if there are only a few seeders, (and none of them have open ports). It’s super frustrating looking at a torrent with 3 or 4 seeders, but seeing that it has stalled.

There are workarounds like pinholing, but those are bodges that require specific circumstances to work.

IIRC it's good to get faster downloads on BitTorrent, but I've never stumbled upon a torrent that was slow anyway.

I haven't even looked at port forwarding. I think maybe some ISPs in some countries might block traffic this way but others will know better. Everything just works with Mullvad on Fedora with Qbittorrent, so I'm happy.