As GoanGeek said, power saving mode could slow notifications from arriving. If that's not switched on, there's a few other things to try: the app you're wanting the notification from may have been put to sleep, if you check in settings, apps, then your chosen app, then Battery and try changing the setting to Unrestricted or Optimised (if it was on Restricted).
There's a few other things you could tweak to make sure the applications you need to send notifications are prioritised, but try those options first and see how it goes
