Seeding without port forwarding is really slow IMO because often you’re seeding to someone who is mostly a seeder themselves. Most leechers have it disabled so they can’t really establish a connection in either direction between themselves AFAIK, putting extra pressure on the seeders with open ports
I moved away from using an older Pi for this because of the exact same issue 😂 as soon as you’re seeding to any quantity of leechers the thing slows to a crawl with the CPU maxed out