Normally Zing alert notifications do arrive quite quickly, most of the time in less than 2 seconds. The most common reason for notifications being delayed is the phone or device had no internet connection or a poor connection and the notification did not arrive until the connection was made.
However, there are some other cases where we've seen them get delayed:
Delays for notifications on Android Devices:
- Some home and commercial wifi routers will cause the phone's connection to Google's FCM/GCM servers to be closed. The device later re-opens the connection and receives the delayed notifications. (Discussed here: https://groups.google.com/forum/#!topic/android-gcm/Y33c9ib54jY)
- Some custom versions of the Android OS have a power-saving setting that can cause notifications to be delayed or ignored. For instance, Sony Xperia devices have a "Stamina Mode". (Discussed here: https://talk.sonymobile.com/t5/Xperia-Z3-Compact/Notifications-not-Working-When-Phone-in-Sleep-Mode/td-p/879641)
- Some Android devices can enter a low power mode when their battery is low that may cause slight delays in receiving notifications.
Delays for notifications on iOS devices:
- As with Android, some wifi routers will cause iOS notification delivery delays.
- Notifications may be delayed if "Restrict background data" is checked for the app, if Wifi-Optimization is turned on in the advanced settings, or if priority mode is enabled.
- "Low Power Mode" is enabled. If in your Settings > Battery > Low Power Mode is enabled, this will reduce the rate at which notifications can be received or turn them off completely.
WhatsApp has a good list of instructions on troubleshooting delayed Android and iOS notification at the bottom of their FAQ page here: https://www.whatsapp.com/faq/en/android/20887936