The Menu is slow, how to find out why?

[Peter]

Hi folks,

  I have a strange problem: the menu takes 1-2 seconds to open.

I have read that one shouldn't use the menu, and I might agree,
but: my personal keybindungs have come a long way, and they are
very different to the default ones. So, whenever I do something
with my configuration files, and loading fails at first, I am lost,
and then the menu should come to help.
And then it goes like this: ups, save&exit doesnt work, wait,
use <f10> |wait 2 seconds|, find "Save", ups that doesn't exit yet,
use <f10> |wait 2 seconds|, find "Quit". And so on...

In such situations the 2 seconds get very annoying. And I think
they shouldn't be there, because
 - they happen only with GTK3, not with MOTIF, not with GTK2.
 - they happen only on one of my machines (desktop), not on the
   other (laptop). The machines should be configured identical,
   they run the same version of FreeBSD 13.2, and (for testing
   currently) the same build of emacs, local configurations are
   removed.
And anyway, if there is such a difference between the two
machines, I certainly want to know why.