I can also confirm this bug, After entering login details in SDDM, plasma splash screen appears but the progress bar remains frozen at 0%. During that time, from a console using `top` I can see kdeinit5 running at 100% CPU. After a few minutes the plasma session finally opens normally.
Loopback interface and other network interfaces are on. My system is debian sid amd64, with intel CPU and intel graphics. I'm standing by to provide any further information required. Kind regards, Theodore
