It’s hard to tell from a verbal description. It could be as easy as wifi interference, either inside your network, of affecting your network from outside. But it could be your Mac’s hard drive sleeping, in a low power state, taking time to spin up and respond to the Sonos speaker’s request for data, and then passing that data back to the controller. Worth double checking the Mac’s power settings preferences have it set to be always on, rather than just wake on LAN request, it might help.

At the end of the day, it might show up in a diagnostic, if you took it within10 minutes of experiencing one of these slow responses, or at least some partial data, since the issue likely isn’t in the Sonos software, but how other things are responding to the Sonos requests. Post the diagnostic number here for the forum moderators to pick up, or call Sonos Support directly to discuss it.

When you speak directly to the phone folks, there are more options available for them to assist you.

A controller accesses the library via the 'Associated Product' device. This can mean several hops across the wireless network. If it's sluggish due to poor signal strength or interference those effects will be felt. 

Much appreciated all. I’ll try calling customer care to help diagnose. I don’t have flexibility to move the iMac itself to a location to wire to a router but a small NAS is feasible. 
 

the diagnostic number is 531911045. I ran this before first play this AM

From your Associated Product IP address (in About My System) you can trace the path a library artwork request would have to make. From the controller to the AssocProd via the router. From the AssocProd to the Mac via the router. Then back again with the requested data. At least 8 hops.

In my experience the number and quality of wireless hops can make a big difference to artwork response time.

Almost certainly a NAS wired to the router would improve things.

Something you could try, before trying Sonos access the drive your music library is on from another device, then from Sonos. If that works it may be a sleep mode issue with the music drive.

I moved to SSD storage for my music to avoid just that. You don’t need a fast or fancy SSD drive for music so a good quality but inexpensive one will work.

Thank you very much - it seems to be a slight combination of all the above! I did a traceroute from the controller to each of the S1s and it’s a long route with latency. I tried putting a couple of mp3s on a flash drive and using the USB port on my router and all worked fine. 
 

if a follow-up question is ok/not annoying, if I go the NAS route (removing the wifi interference issue) is there some way of determining the impact of going to regular drives? The SSD options are too expensive and this will primarily serve music. Most of my collection is .mp3 with a few FLAC and .WAV. I’m hoping to keep this venture below $500 CAD if I can

The only difference when using a conventional HDD instead of an SSD is that Sonos might glitch on the first song if the disk spins down when idle. Disk transfer speeds are orders of magnitude greater than that required for music streaming, so will never be an issue.

As for a NAS which spins down, an old trick to avoid a timeout on playing the first track is to simply browse the music library in the controller first. When artwork appears the disk is up and running. 

Getting a drive designed for NAS use and disabling the spin-down is a decent option.

SSDs aren’t really expensive, they can look that way if you don’t factor in the Sonos limitations.

-- The music library is of a very limited size, 65K tracks or less.

-- Sonos has very limited data needs so the slowest of SSDs are fine.

-- Sonos access is mostly reading so you don’t need high endurance SSDs.

I way over the Sonos requirements using a $35 Raspberry Pi as my SMB v1 server and a $40 250 GB SSD stick for storage. A thumb drive would do as well and cheaper.

I only use 130 GB to store my music and album art for 460 albums of CD quality FLAC music.