FiiO removed the 5800 limit long ago. However, the more songs you add to the media library the slower the device responds, as expected beyond a certain point. From what I recall from other posts the slowdown become noticeable around 20000 tracks.
Alternatively you can browse by folders. My folders are organized alphabetically, A, B, C, D, E, etc., and below that is the Artist folder, then the Album folders with the corresponding tracks. Makes navigation very easy, and the Play Through Folders function keeps the music playing constantly.
Also, IMO, it would be easier to load multiple mSD cards to fit your gigantic library than to carry multiple devices.
I completely second multiple mSD cards.
Another idea for how to speed up browsing a lot.
I have a quite large library. But I browse by folders, because I don't tag properly every time.
What do I do: I name folders by genera, not by letters. My folder structure looks like : Heavy Thrash / Metallica / 1997 - Reload / 01.Metallica - Fuel.flac
Now, the main advantage is that after I managed and ordered the playlist personally, I do get good coherency than when I tried with one letter. Another trick is that I don't have to use generas as they are. For example, I have a few more rough bands, like Wintersun, The Absence, and Utsu-P, under a single folder named death metal, just because all of them give off a rough and energic vibe that fits all of their moods.
The point is that folder structure can be managed in many ways, the way by which someone finds what they are looking for is the best!