FWIW I am running Sigil on Arch as well, using Cinnamon-desktop.
Sigil is installed from the repos, but I don't see why that should make a difference one way or another. Arch PKGBUILDs should be doing the exact same process -- no patches.
I am suffering no problems or slowdowns. Impressive, considering this laptop is ~10 yrs old.
Perhaps KDE+Arch+Sigil are combining together in a weird way.
I am not willing to install KDE to test the theory though.