Added second HomePod to form stereo pair - now very slow to connect

I've had a singe HomePod since release that 95% of time connected instantly when selecting it on my phone as the audio output.

Recently i bought a second HomePod and set it up as a stereo pair with the first. Now, rather than connecting instantly, it takes 10 to 20 seconds for them to connect and start playing when I select them. I also get a very long pause when i pause/restart on the phone or try and skip forward 30 s in the podcast app. All this occurred instantly previously with the single HomePod. This is not intermittent - it is now always terrible.

Factory resets have been performed on both HomePods. They are both connected to an AirPort Extreme basestation, with the connection between both Homepods and the basestation being described as "Excellent" in the iOS Airport utility.