Hello again,
Extremely slow opening new gtk4 windows, or even a file chooser dialog (very slow, once oddly even crashed it, open file chooser, no action yet) ?!?! This does not happen with gtk3. Gtk3 apps and file dialogs are fast and snappy.
This is a long standing mission critical Debian system, been updated multiple times. And I a not really in a situation time wise the reinstall the whole beast as running many customized unrelated services.
I am still having this issue I started a while ago (see link below to), but new insights as non of the X11 rending/graphics related ideas helped and also are not the actual problem.
BUT:
I found out that the issue does not occur on the same old system when running the same larger application from a newly created user account!
I can assure it is not related to the application settings/configuration itself.
But I fear, even can not confirm easy, some thing related to dconf/settings/gtk4 user account stored configurations must be the cause?
Any ideas or hints where to start looking?
As again, I do not want to wipe all old accounts settings blindly 
But understand and fix it right.
