I found the deluge (web?) ui becoming unusable after adding tens (or hundreds?) of thousands of torrents.
Not sure about the details, but a decade ago I used to seed all files below 100MB on many private trackers for seed bonus points, and yea, deluge ui (might have been the web ui, not sure) became very slow. :D
Same, deluge and qbittorrent would start to have issues with very large or lots of torrents. Ended up with transmission with the trguiNG UI and its handled everything. It's not perfect and often slow but it hasn't crashed.
I ran into slowdowns in the remote control after just a few hundred. I switched to transmission shortly after. I had a great time using Deluge for probably like 6-7 years but Transmission is more performant has more tooling support.
