Android 8 delayed notifications
Here's a list of things you can try:


Make sure you haven't disabled the app by force quitting on it
Make sure power saving mode isn't on 
Check Data usage to see if Background Data is enabled for the App
Turn off battery optimization from Battery Usage option for the App
Make sure that there isn't any third party overlayer that you've utilized which is utilizing the app sync permissions ( certain launchers/ addons may do so)
Clear cache for that particular App 
Factory reset the Google app by selecting the Uninstall updates option
Install this app and set your heartbeat rate to 5 minutes. It's a quick fix for the issue. It's most probably a network driver issue which the company needs to patch up and in the meantime, you can use this app to ensure normal functionality