Actually that index does not speed up the total artist list.
It speeds up displaying all albums/songs by a distinct artist.
Anywho, you might want to use the nightly versions of firefly, since there is constant improvement, as opposed to the “stable” releases.
Removing the DAAP plugin would only save you a little memory on your NAS device, but not speed up performance in general. Which device are you running firefly on, anyway?
Always interesting to see what boxes are capable of doing so. 🙂