I found Firefox was taking a while to launch in XFCE4. I tested removing the plugins, emptying the cache, etc. but nothing worked.
Eventually I ran "strace Firefox" and found it was hanging polling something for 25 seconds.
So armed with the readout from strace, I searched and found the solution :
sudo pacman -Rndd xdg-desktop-portal
I kept xdg-desktop-portal-gtk as the bug isn't in there. Apparently xdg-desktop-portal-gnome has the same bug, but I don't use Gnome so...
Both Firefox and Libreoffice now launch in milliseconds!
I hope this helps someone else...