After some time-consuming on the problem, I have notice that there are a flatpak folder exists on the .local/share/flatpak path. Although I do not have flatpak installed.
Inside the flatpak folders there was an empty db/documents file.
I guess it was installed alongside some packages I was installed.
In my case, removing the .local/share/flatpak folder and restarting the session fix the problem.
After that snap apps run just in a second. (Tested on Termius and Flameshot)
Warning: Make sure you have a backup before you do that.
Apologies @AliBaghernejad - yes it is a snap install of Chromium. I have previously used (non Snap) Chromium for many years without issues (same machine, older version of Ubuntu).
The snap chromium was recently installed on a stock Ubuntu 20.04, worked for a week or 2 then out of the blue seeing ‘font-feature-settings’ is not a valid property name.
I’m not seeing the same issue as the OP and not intending to hijack their thread, I just saw that we are getting similar error messages for snap chromium installs, on Ubuntu 20.04, and within a few days of each other.
I have uninstalled and reinstalled the snap and now working at present. Fingers crossed
Encountered the problem after reinstalling snap-store with wayland gnome. Deleting the ~/.local/share/flatpak folder solved it, even though I have no flatpak installed. However, after restart Firefox immediately recreated it for an extension companion app. Did not impact Firefox launch time though. So it was probably a permission error.
(Tested on Termius and Flameshot)