I think Transmission is a very competent client. However, when I last used it a few years back, as soon as you reach 1800+ seeded torrents it really struggles. I switched to rTorrent but experienced similar issues at around 2500+ torrents. After hours of tinkering (which rTorrent / ruTorrent does have a lot to tinker!) and not getting anywhere, I made the switch to qBittorrent which is still happily chugging along seeding 4500+ torrents.