Plasma Workspace 5.12.5 has an issue when compiled with ki18n 5.45, cmake can get confused if your build path is long and hang until it runs out of memory.
Should be fixed by Diff 12216 "Don't need to run previous iterations commands again" [Closed] https://phabricator.kde.org/D12216 https://cgit.kde.org/ki18n.git/commit/?id=9c32bdab29b345976eee3b9c1c4bebd76cdcdde2 Jonathan