error "client bug: event processing lagging behind by ..ms, your system is too slow"

On my two Debian/unstable machines with the libinput10 1.19.3-1 package (but the bug is older), I get disturbing errors of the form "client bug: event processing lagging behind by ..ms, your system is too slow" in the /var/log/Xorg.0.log log file:

[    86.305] (EE) event5  - PixArt USB Optical Mouse: client bug: event processing lagging behind by 13ms, your system is too slow
[   110.541] (EE) event5  - PixArt USB Optical Mouse: client bug: event processing lagging behind by 16ms, your system is too slow
[...]
[ 20438.855] (EE) event3  - Apple, Inc Apple Keyboard: client bug: event processing lagging behind by 19ms, your system is too slow
[ 22425.372] (EE) event3  - Apple, Inc Apple Keyboard: client bug: event processing lagging behind by 20ms, your system is too slow
[...]

and

[   200.076] (EE) event2  - DELL Dell USB Entry Keyboard: client bug: event processing lagging behind by 11ms, your system is too slow
[  1778.975] (EE) event2  - DELL Dell USB Entry Keyboard: client bug: event processing lagging behind by 14ms, your system is too slow
[...]

Debian bug report (by another user): https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=968093

The compositor is mentioned at https://askubuntu.com/a/1292837/691424 and https://forums.gentoo.org/viewtopic-t-1117556-start-0.html but I do not use a compositor (I don't use GNOME either).

The FAQ says:

When this warning appears, it simply means that too much time has passed between the event occurring and the current time. In almost all cases this is an indication of the caller being overloaded and not handling events as speedily as required.

But my systems are not overloaded (they are idle or almost idle). And the fact that such messages are seen by various users suggests that this is obtained in normal use.

BTW, in "client bug", it is not clear what the client is.