Why is there a delay between mail arriving and notification?

I see this question was posed back in 2018 (see: https://discussions.apple.com/thread/8262026).

It's now 2020 and the problem is still there even using Catalina 10.15.6.

Actually, I see no notification at all, no banner, no sound, no bouncing icon in the dock. Nada until "sometime" later.

It's really a nuisance to be notified of new mail sometimes 30 minutes after it's arrived!

Is there a fix or do we really have to wait for macOS 11 Big Sur, and will it even be fixed then?