Seems odd, none of my newer devices had any impact on my WiFi speed, but perhaps we have different types of networks. Have you tried to call Sonos Support to discuss it?

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

Describe your network for us. Are any of your SONOS units wired to the network? Are you using any “managed” network switches? You could be experiencing a “network storm”.  Make sure that the switches are configured to use “STP”, don’t use “RSTP”.

If you are the victim of a storm, immediately after starting or restarting the network it will run at normal speed, but after a relatively short while, as a storm begins to build, the speed will decrease to a crawl as a blizzard of packets begin to build.

That can happen if your Sonos speakers are creating interference or running on the same channel as your Wi-Fi. Try switching your router to a different 2.4 GHz channel or wiring one Sonos to Ethernet so it forms its own SonosNet, that usually fixes the slowdown.

Whatever AI you are using is incorrect.  “Running on the same channel as your WiFI’ could only mean you are already running SonosNet, so the subsequent advice to wire one Sonos to Ethernet “so it forms its own SonosNet” contradicts that fact. 

Also, as @buzz stated, the most common cause of a network slowdown is an STP network storm, which is caused by having more than one wired device and a network not configured for STP.  Wiring yet another Sonos device will exacerbate that problem. 

OP states that “New Sonos devices” drastically reduces their WIFI speed. Now Sonos devices are not compatible with SonosNet. While hardwiring a few speakers, regardless if your router and services are fine could be a solution. 

Hardwiring a main speaker like a soundbar or a Five could help if there is some interference affecting speaker connection.

Thanks so much. I turned off the wifi on my amps and that helped a bit (can still use app). Then the technician said he has a lot of issues with new appliances. Sure enough, I have a new smart range, 3 refrigerators and wine cooler that all use wifi. Turned them off. AOK

You need a router upgrade - a decent router should have no trouble with this.

I have around 60 devices on my wifi, on various frequencies, wifi standards and two VLANs. 14 are Sonos devices.