I had a similar issue occur on multiple computers and figured out that for some reason the portal implementations are not started properly, so the main xdg-desktop-portal process cannot export their functionality, which creates this delay.
One possible temporary workaround is to manually start the implementations (depending on whether you have installed it, either from ~/.nix-profile/libexec/xdg-desktop-portal-* in case of a user level package or their respective store entries if they are declared in your configuration.nix) and restart the main xdg-desktop-portal service afterwards, which will be happy now. (You can also try manually starting it in verbose mode and observe either the success or the 25 seconds of timeout that occurs while waiting for implementations to be spun up.)
Feel free to ask for clarification if it isn’t clear enough and I’ll try to explain what I mean in more details.