Remember, you’re dealing with the internet connection between your speakers and your router, so the speed of the router to outside sources (that fiber 1Gb) doesn’t normally play in to this issue.
That being said, you haven’t told us anything about your internal network setup. Are the speaker on your WiFi directly, or is at least one device wired. If they’re on your WiFi, it could easily be an issue with wifi interference , I’d encourage you to read the linked FAQ and apply as many of those solutions as possible.
Another possibility, if your devices are either wired or on your WiFi, is simple IP address duplication, caused by your router handing out ‘bad’ IP addresses every time the Sonos reboots at every software update. This is easily tested by unplugging all Sonos devices, then rebooting your router. Give the router a couple of minutes to reload its software, then plug back in your Sonos devices. If this ‘fixes’ the issue, I’d encourage you to set reserved IP addresses in your router for at least your Sonos devices. Instructions on that process are usually in the router’s manual. I’ve done this, several years ago, which stopped my issues with dropouts.
