I'm reading a lot on internet, search for "
Android notification delay" and you will see tons of threads of people struggling for this issue.
Yes, the problem is related to battery optimiziation and DOZE mode, introduced in the lasts Android versions.
You fill find a lot of hints on how to disable this feature but (in my case) they don't work.
This is a good place where everything is well summarized, if you want to give it a try.
Definitely, you must disable DOZE mode if you want instant notifications as before, but the only method that seems to work is through ADB (and need to be repeated everytime you restart the device). If you want to try, follow
here (you can revert back without problems then). I've just tried and it works perfect, but I can't say if it has an impact on battery life (I suppose yes).
There is also a 5$ app on PlayStore to easily disable DOZE mode, I didn't try it yet.
I'm disappointed: if you navigate in settings menù there are all the options to disable doze (also for specific apps only), to allow data in background etc. But they simply has no effect. How is it possible ? I understand the goal for a manufacturer is to maximize battery life, but why don' give the user the option to choose what to do ?
In our days (and in my opinion) instant notifications are mandatory. What fucking notification is if you are informed of a motion detection, an alarm, a simple message also, after 5 minutes ?