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 methods are to 

  1. wait for the message to arrive on the handset if the device is in your control, or 
  2. use secondary methods like our Shortlink feature 

and measure this across a appropriately sized target group and time.