EDIT: I still have problem, so I uncheck this as “solution”
I’ve update system and have render issue. An icon on the task bar shows message “Software renderer in use”, and KDE became unusable.
I found out that the culprit is the lines in the config file ~/.config/kdeglobals:
[QtQuickRendererSettings]
SceneGraphBackend=software
After delete those lines, I notice that Krusader seems to be more responsive now.