Dave has noticed sometimes his text messages take up to 15 minutes or more to be delivered. Leo says that since Dave is using Verizon, they are using an old system called SMS, which uses cellular to transmit, rather than data. They use it because it's extremely lucrative, rather than using data-based schemes. That's why Apple has bypassed the carrier infrastructure with iMessage.
Dave could change carriers, or he could change numbers and get something like Google Voice. Then the text can be sent to up to five different numbers. It's pretty quick, even though it's using another gateway. It will also email him.