iTunes uses the DAAP protocol to list the songs.
The DAAP protocol assembles a ‘file’ to send. The first 8 bytes mention the size of the file being sent. This means the file has to be complete.
f.e.
When listing for songtitle and artist, the file is assembled. When done, the size is known, and the file can be sent. So it first has to index all music before being able to start sending the file to list all songs using DAAP.
The soundbridge uses the RSP (Roku Server(?) Protocol) protocol. This is a new protocol implemented from around 1400, where the file to list all songs does not need to know the size. So as soon as a listing is requested, the first indexed songs are sent, while in the background, the rest of the songs are being indexed.
In short:
DAAP has to index all files, then send the list.
RSP can index and send the list at the same time, being more faster as the database gets bigger.
Hope this clarifies the situation. 🙂