Have you asked Apple?

Sonos simply provides the interface to the service. How the service performs is down to the service itself.

Slow starting on the Sonos app? I spent some time with Sonos Support and found most of it was caused by issues with my positioning my Sonos speakers too close to each other. Moving them a foot or two got rid of most of the delay.

A bit of the delay was from other wireless interference with one of my other speakers. No good way to deal with that as I really like that speaker where it is and moving the interfering device would not be good either. So I live with the small delay until I can replace the noisy device.

If it is Apple Music that is slow to start I can’t help you, but my Amazon Music starts in a second or two once the App is fully awake.

Which search are you using in the app, the old or new?

What I mean is that everything in the Sonos app is slow and the search sucks. It often doesn't find what I'm looking for, whereas in the Apple Music app it finds it right away.

Sonos should work on that.

Except Sonos merely sends the request to the Apple servers, then displays what is returned. Sonos doesn’t  do any extra processing there, as @ratty said earlier. 

The suggestion that the ‘app is slow’ does suggest some network issues, though. Anything from wifi interference , to duplicate IP addresses, to almost anything else, which can’t be determined from what Details you’ve posted. I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

Then try to find something in Apple Music and in the Apple Music app on Sonos. The search results won't be the same.

The Apple Music native App will always be superior to the Apple Music Service in Sonos.

Could you find your music in Apple Music and add to playlist, eg “Sonos Atmos”, and then find the music on that playlist on the Apple Music Service in the Sonos App? Yes extra steps, but maybe quicker overall than using the Sonos search?

It's annoying.
Up until now, I've been playing music through Spotify on my Bose speakers.
 
Now I'm playing it on Sonos too, except the volume is adjusted with the buttons on my phone in a weird way. I press the minus button and the volume slider goes back to the original place.

There is definitely a problem with the Apple integration; as we don’t know how it is implemented it may well be on Sonos side, Apple side, or both.

I’ve heard this issue from a multitude of people that use Apple Music with Sonos, so let’s not do a ‘just log a ticket’ here, there is definitely a problem with this service that is not user specific.

The app is very slow to start playing songs on Apple music and often fails; when you press ‘play’ I often see the spinning wheel for up to 15 seconds. Then sometimes it starts playing, sometimes it just adds the song to the queue without starting and often it just gives an error message. Sometimes I need to repeat this up to 3 times to get any music going.

My house has enterprise grade WiFi with all sorts of diagnostics running and I don’t have this issue with any other network music services, so it’s very unlikely that this is a network issue. (Spotify, Amazon, BBC Sounds all play instantly).

I have the exact same issue in my other house, again no issues with other services.

I have a feeling Apple has put a lot more security/authentication on their services than others (only service that requires activation in their iOS app), so maybe that is a contributing factor, maybe their faffing around with certificates and the way the integration is set up may interfere with this. Pure speculation on my part.

Not sure if Sonos is to blame, but their engineers should definitely have a chat with Apple’s engineers.

We absolutely, 100% know how the interface to Apple Music is implemented, and it is on Apple.  All Sonos does is supply the API.

https://docs.sonos.com/docs/content-service-get-started

Sonos supplies the UI and the player implementation; Apple likely provides some sort of streaming backend with an API.

As Apple controls both the backend and their own front end they undoubtedly have the ability to provide a smoother end user experience. As I work in this field myself, I know that internal implementations often use features or private APIs that are not available to third parties.

Given what I’m seeing I have a feeling there is an authentication issue with Apple’s API, but Sonos’ engineers would be much better placed to investigate that.

Pointing finger at each other is not going to fix it.

Honestly, you couldnt be more wrong.  Read the link.  Sonos supplies the API, Apple does the implementing.  It's all there in black and white.  And pointing fingers most certainly will get it fixed, if you point it at the company tasked with fixing it, and that isn't Sonos.

Authentication has to be done by Apple: think about it, Sonos don’t have access to the Apple user database. All Sonos have is the access token and refresh token they got from Apple when the user registered Apple Music on their system. Those tokens are included in the SMAPI calls that the Sonos app makese to Apple’s service, its up to Apple to authenticate with those. You can read about it here: https://docs.sonos.com/docs/use-authentication-tokens