Konsole very sluggish, sometimes lags very long before updating screen/take focus

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1

Unfortunately, after I switched from FreeBSD to Linux, the behavior of Konsole has become extremely sluggish.
I am using FVWM, as the KDE plasma is too weak to serve a total of 80 screens on 5 desktops, and is also not capable to manage windows larger than the screen.
So there is a full KDE installed, and the KDE apps run under FVWM.
However, Konsole’s behaviour is unacceptable.
When going to a screen which contains Konsole windows, first it looks as if Konsole is hung/unresponsive: it does not update its window, so there are the contents of the screen I was coming from.

The delay one has to wait until Konsole becomes responsive and takes the focus varies between almost instant to 2, 3 minutes.

There is no swap involved, the swap partition is not used, swappiness is set to 1. top says there are more than 100GB of RAM free.
System load is near to idle.

.xsession contains this:
setxkbmap -model pc104 -layout us -variant alt-intl -option “”
# comment out to turn on xorg’s default screensaver
xset -dpms s off
# exec dbus-launch --exit-with-session ~/.xinitrc “$@”
# start up KDE runtime stuff
kbuildsycoca5 --noincremental
kded5 &
# FVWM
exec fvwm3

The system does not use “graphical login”, wayland or similar trash.
I Just run startx.

The described bad behaviour does not occur on FreeBSD.
It only occurs on Linux, both Debian and Ubuntu.

So, what is wrong, what makes Konsole behave this badly on Linux?

2

What a pity there is no answer yet!

I am suspecting somehow that dbus might the problems’ root cause.
Is there any command line argument option for Konsole to not use dbus?

Honestly, I hope this problem is solvable.
Otherwise I’ll have to use urxvt or any other terminal emulator, as Konsole is the only program on this computer which shows that unresponsivity issue.