Does the rest of the system start quickly? Concerned you may have a failing drive. I’d check dmesg logs to see if you have any messages during that time.
Thunar has been the default filemanager on Openbox ISO and I know there is (has been) infrequent issues with Thunar being slow to start.
With no intentionally relation to the above - but more as an effort to create a true minimal ISO the most recent version of Openbox ISO has PCManFM (gtk3) onboard.
I have not had any issues for a long time neither using Thunar nor PCManFM.
I had installed gnome-builder the other day, which installed flatpak which installed xdg-desktop-portal-gtk. I’ve removed everything (coz dependencies) and my system is usable again.
For less drastic solution add te following to .xinitrc or .xsession