Nobody from the network's ever stated text messaging was meant to be an instant service, its just because everyone is used to sending a message & it arriving at the other end in under a minute that they expect it to be instant & moan when a text gets delayed.
Sometimes due to network issues it can be up to 72 hours before a text message gets through to the other person.
Ive had text's delayed for a few hours from time to time, it's just how it goes with mobile phone technology every so often.