Indexing Music Library

As a guess, it may have to do with the CPU doing the processing, in conjunction with available RAM, where it may be doing swapping. Scanning data is certainly a different operation than a backup. 

How many of the internal tags does your backup process?

How many indexes does it generate so you can search your music based on them?

How many cores, how much speed, how much memory in your backup device versus your Sonos?

Might go faster if you did the indexing from your most capable Sonos device. 

Does Sonos choose the “Associated Product” ( The product that takes the index ) by looking for the most qualified product to execute that function?

Been a long time since I saw the Sonos position on this.

Originally it was the associated device, then I think it was tweaked so that if the associated device was underpowered it would move the indexing to a  more powerful device. I don't recall any details on when the switch was done. Also haven't seen anything recently so it may not be in the new app.

Edit, also don't know how you'd tell where it was being done, maybe feel for the warmest one? :-)

How large is your library? Where is it stored? Are your players wired or wireless?

Probably there is some network congestion. Each track must be fetched from storage by the player running the index and processed by the player. Eventually a copy of the index will be sent to each player. A first build of a new index takes longer, adding or updating a few tracks is much faster.