Are you intentionally avoiding all the times people have pointed out
xdg-desktop-portal-gnome
?

See wiki-post:
philm:
Many applications (firefox, thunderbird, etc) slow to start on desktops other than Gnome

My XFCE had the tortoise-paced firefox & telegram startups after this. But Philm’s solution worked.

I noticed both my browsers (Librewolf/Firefox, the latter being a flatpak) and applications like Screenshot, Libreoffice and Gnome-System-Monitor being incredibly slow and unresponsive

Once again … please see the wiki post and attached reference to bug in xdg-desktop-portal

You may want to check if xdg-desktop-portal-gnome is installed and remove it. It is stated in troubleshoots.

Oh come now …
Its been stated multiple times …
(including in the post your quoted/responded to)
check the wiki post.
xdg-desktop-portal-gnome
bug
… I might have missed a few …