Why am I getting delayed alerts

[AC Question]

I'm using an internal app (which does not receive push notification - To-Do lists for Google by Pselis Ltd.), and in the last few days, my reminder alerts became delayed by 4 minutes. I'm using a Samsung Galaxy S5 Neo, running Android 6.0.1. I've uninstalled all apps which I've downloaded recently in case they may be conflicting - this hasn't helped. I've uninstalled the To-Do list app and re-installed it - this hasn't helped. On the chance that it might be receiving Push Notifications, I installed the Push Notification Fixer - this didn't work. The app is NOT optimized. I tried installing another similar To-Do List app - G-Tasks and had the same problem.

Any help you can give would be greatly appreciated.