I have a few dodgy cheap SSDs, so I'm not surprised by slow I/O performance in general. But my Plasma desktop frequently freezes for 1-3 seconds (including the cursor!) whenever there is high IO load, for example while installing a Steam game.
Updating the screen should be completely independent from I/O, shouldn't it? Is there anything I could've misconfigured in my Arch Linux? I'm running Plasma 6.1.4 in Wayland.
Try searching it on https://btdig.com/, a DHT search engine. Most of the torrents there are very old and have barely any seeds. Be patient. The last thing I downloaded from there took 1-2 weeks to complete.