For context, I have the Nest Doorbell Battery but it is wired to mains and not running power saving to increase the update frequency. After struggling with notification latency and factory resetting many times, analysing wifi RF environment and pulling of many hairs, I've found the solution (in my case at least) to the delayed notification.
Background: When doorbell was new or newly reset, notifications would be near instant. After a period of months, things would gradually slow down, to the point whereby whoever rang the bell and waited a short while would then walk off and be down the drive before my Google displays and speakers announced that someone was at the door - rendering the doorbell absolutely useless. Factory reset the doorbell and all good to go again for a few months more. Rinse and repeat ad infinitum.
The cause: After trial and error, I've found the issue to be facial recognition, partially down to the volume of facial snapshots you have stored for each person and/or number of different persons you wish to recognise.
Solution: Ideally, if the speed of notification is critical, leave facial identification off and all will be well. If you have a requirement to identify people before answering, for security or personal reasons, then I would advise to keep this to an absolute minimum so that the doorbell/cloud server doesn't have to chew through a large number of facial profiles before returning a result, therefore delaying the chime and notification.
Hope this helps some of you.
