Also, with DTC there is a continuous net settlement system that basically nets a brokers transactions together for the day and basically lets them know which securities are credited or debited from their account and how much they need to pay or receive for the day. It helps solve the issue of high speed trading and nets each brokers obligations out at the end of each day. In its simplest form if a broker had two customers and Customer A sold a share of AAPL and Customer B bought 2 shares of AAPL. The CNS system would basically net those two transactions together and calculates that the broker is going to receive 1 share and that they will pay x amount of dollars.

There is definitely room for improvement because sales don't settle till T+3 (however the settlement cycle is shortening to T+2 as of Sept 5th)