Push notifications rely on services like Firebase Cloud Messaging (FCM) and Apple Push Notification Service (APNs) to deliver messages to users' devices, either in the app's foreground or background. For iOS, notifications can be restricted unless the app has specific permissions, while Android may delay notifications due to battery optimization features. To implement push notifications, developers collect device tokens, set up a backend server, and authenticate using API keys or certificates. Integrating PubNub's Mobile Push Gateway with services like FCM and APNs enhances delivery, even when users are offline. Developers must manage device token registrations, handle errors, and monitor delivery logs to ensure reliable notifications. They should also consider potential delays due to API limits and implement strategies to mitigate these issues, such as retry mechanisms and batching strategies. PubNub offers low-latency delivery, with most messages arriving in under 20 milliseconds, though developers can request higher throughput if needed.