How long does it take to send a message to WhatsApp?
Messages sent to WhatsApp via the Chat API are received instantly.
Possible delays may be related to the speed and quality of the Internet connection. It is best to connect your phone to a stable Wi-Fi network.
Also, with the departure of the WhatsApp application on your phone in the background mode, there may be interruptions in sending messages. Make sure that the phone does not go into the background mode, the easiest way to do this is to leave WhatsApp the only active application.
There are also delays in processing messages from WhatsApp itself. During the messenger update periods, messages may not be sent, but they are all stored in the queue for sending.