What you’re all forgetting is that if you action a request from an iPhone and not the HomePod then it all works fine.Its very strange. I wonder if its something related with something they changed in the homekit code for IOS 15.
I´ve run homemade tests against homepods on 14.6, and its puzzling. Homekit works perfectly and consistently on the homepods with 14.6, which tells me is not a serverside issue, its software related. On the Homepods with 15.0 or higher, its a disaster.
The interesting thing is that the erratic behavior seems to affect to several brands of accessories, so there are not compatibility issues with one specific manufacturer.
Either Siri on the iPhone or a Shortcut on the iPhone or a Scene from the iPhone and it’s instant.
This is very much a HomePod issue.
Please do try this and report back.