Why the first sync takes so long
It reads your entire catalogue once. After that, browsing never touches the network.
The first sync copies your server's whole catalogue (every album, artist, track and cover reference) onto the device. On a large library that is tens or hundreds of thousands of rows over Wi-Fi, and it is genuinely slow the first time. A library of ~950,000 tracks has been tested and works, but it is not quick to import.
The payoff is that browsing afterwards never touches the network at all. Search, sort and scroll read a local database, which is why they stay instant on libraries that would make a network browser unusable.
After the first sync, later syncs only bring in what is new or changed. A top-up takes seconds.
Two things help: keep the app in the foreground for the first run if you can, and let it finish. If it is interrupted, it resumes from where it stopped rather than starting over.
Still stuck
Write to us with your server, your streamer and what you expected to happen. If you can, send the diagnostic log from Settings, it usually answers the question faster than a description can.