Aircraft boarding times in the US have significantly increased over the years and, in fact, priority boarding has become one of the most sought after perks of air travel. The first to board have choices for overhead storage and are not forced to stand for what seems like forever, while a person stands in the aisle and tries to force a huge carry-on bag into a tiny overhead slot.
Most major airlines such as American Airlines, United, Delta and US Airways as well as low-cost carriers such as JetBlue, AirTran and Frontier Airlines have adopted a zone boarding method with variations. Generally, this includes pre-boarding of elite/premium class passengers followed by zoned general boarding. Southwest is an exception and has adopted a unique boarding-groups based boarding system with four assigned boarding groups (A,B,C,D).
Read more details on the boarding procedures of US airlines at:
A Look at How Different Airlines Board Their Planes