Apply the steps on https://dontkillmyapp.com/ to Google Play Services and Google Play Messaging, that may help. Also check if you have an app running in the background that eats all of your RAM and forces the OS to clear up scheduleable tasks.
Thirty minutes is way longer of a delay than I've ever seen. You may want to try a factory reset in case it's a ROM issue. Fairphone has had its ups and downs over the years when it comes to the built in software.