It sounds like some kind of power saving feature has been turned on, where your phone only connects to the internet at some set interval. Then when it makes it grab it gets all of the late notifications... Are you using a power saver app? Have you checked your native power saver settings? Usually it's app dependent but check your sync settings too... I can't remember if in Oreo there's an option to sync only over WiFi that covers more than individual apps...? It sounds like you're getting the right now stuff because you didn't mention SMS or email, just apps. You might also check app priority. I do remember being able to set app priority levels. Can not remember if that was for notifications or RAM but it's something to look at JIC.