I am having the same issues. Since iOS 15 HomeKit is severely broken. No update yet fixed the issue entirely.
It is slow, cameras fail from time to time, HomeKit Secure Video is a fail with notifications coming after people have left again, automations won't trigger from time to time, automations won't play sounds on the HomePods and so on.
Usually it is getting worse, when HomePods are the active hub.
I don't know why I can't chose the hub manually. For Apple TVs and iPads, you can deactivate them as hubs. But not for HomePods. Unfortunately, those are the slowest devices causing most of the issues. How does this make sense?
So I continuously reboot HomePods for the AppleTVs to become the active hub just for HomeKit to switch back after a few days. This is a complete fail.
I had Apple Support on the phone twice based on tickets. It's ridiculous. They told me, I have to uninstall my OpenVPN Client on iPhone. It appears, they don't understand the difference between "Home App on iPhone not working" and "HomeKit automations not working". Won't waste any more time with this support. Hopefully frustrated customers will flood the support for Apple to act on this issue. But my hope is not too high. I guess it's a performance issue with the HomePods and not too many users are having homes with more than 150 devices. So they will always consider it an "exception".