Hi there 
I suspect that this is happening because there is no option in the base FilePicker to browse assets recursively.
This is an open issue on Foundry Add `recursive` option to `FilePicker#browse` method 路 Issue #10163 路 foundryvtt/foundryvtt 路 GitHub
This means that module may be doing multiple browse calls to traverse each folder in the asset library, and being rate limited by the Forge API (to avoid server side performance degradation due to excessive calls).
The rate limiting means that the requests are taking longer to complete, which is why it may appear to be taking a very long time.
The Forge API warning is explained here Forge API Rate Monitor and does not actually error the calls out, but is designed to inform the user when rate limiting is happening.
In this case, one would expect the call to eventually complete even with rate limiting in play. If you open your browser dev tools (F12), you may see that subsequent calls are still happening.
I鈥檒l reach out to the Moulinette folks and ask if it may be possible to use the Forge鈥檚 recursive browse option when running on Forge for faster indexing to avoid having to make multiple calls (see also Forge API Rate Monitor)