SOLVED Huge uplift in CPU usage when switching sessions

[LoopBack]

Hello.

I'm running Debian 10.1 on an Intel Core 2 Duo HP laptop.
When I start a new session from an existing one using:-
Menu > Power/Session > Switch User
and then go to log back into the first one [because see this earlier post of mine]
the cpu usage shown by TOP becomes a persistant 100%.
Up until the point that I log back into the first session, there is no uplift in the cpu usage.
The high cpu usage is by:- kwin_x11 for the first session.
Has anyone else had this trouble (& perhaps even fixed it)?

[LoopBack]

I usually put my laptop into standby rather than shutdown;
however, having just brought it up from power-off: the problem has cleared !

[stevepusser]

More troubleshooting for your KDE issue: If you log out and back into KDE when it's at 100%, does that make any difference?