The first thing to make sure is to determine how you measure the delay and whether or not that is a valid measurement method. It is a common misunderstanding that a message has arrived at the phone only when we report back the SENT or DELIVERED status to you. As described in Sending process in detail & measuring sending speed - looking at the time when the delivery status is reported, is not a valid method.

The only reliable method is to wait for the message to arrive on the handset if the device is in your control and measure this across a appropriately sized target group and time.

More info: