Database design of space exchanging time

Our system is often used to send SMS, email and so on, in our design will usually create a tb_outbox table, when the data generated, inserted into the Tb_outbox table, by the timer to read the Tb_outbox data to send, and then modify Tb_ The sending status of the Outbox. Yes, this is the usual practice, but when our SMS, email send frequency and number of large enough, our system will appear performance, table locked and so on. So what are we going to do?

One idea of the following design is as described in the title: space to change time. Personally, I feel that this description is more appropriate: separation of objects ' responsibilities, insert, Update, delete, and so on in different tables. Don't say much nonsense, let's take a look at this design chart:

Diagram: Logic diagram