Airlines are trying to save time by speeding up a part of flying that creates delays even before the plane leaves the gate: the boarding process.

This summer travel season, Delta plans to preload carry-on bags above passengers' seats on some flights. Southwest wants to get families seated together more quickly.

Airlines have tinkered with different boarding systems almost since the days of Orville and Wilbur Wright, who tossed a coin to decide who would fly first aboard their biplane. Plenty of people have offered ideas for improvement, but no perfect method has ever emerged.

Most airlines let first-class and other elite customers board first. After that, some carriers fill the rear rows and work toward the front. Others fill window seats and work toward the aisle. Some use a combination of the two. Airlines have also tried other tricks, like letting people board early if they do not have aisle-clogging carry-on bags.

It's not trivial stuff. With many flights full, anxious passengers know that boarding late means there might not be any room left in the overhead bin.

And it matters to the airlines. Slow boarding creates delays, which mean missed connections, unhappy customers and extra costs.

Researchers from Northern Illinois University once figured that every extra minute that a plane stands idle at the gate adds $30 in costs. About 1 in 4 U.S. flights runs at least 15 minutes late. Multiply that by thousands of flights each day, and it quickly adds up for the industry.

Delta's Early Valet service will offer to have airline employees take carry-on bags at the gate and put them in the bins above assigned seats. The airline wants to see if its own workers can load the bins faster than passengers.

The service began Monday on about two dozen flights, and that number is expected to rise steadily during June, Delta spokeswoman Morgan Durrant said.

Early Valet will be offered through August on some departures from Delta's busiest airports — Atlanta, New York, Los Angeles, Detroit, Minneapolis, Salt Lake City and Seattle.

It will be available only on flights that typically have a high number of vacationers. Presumably, business travelers know how to board a plane efficiently. Specially tagged bags will be stowed on the plane before boarding begins, Durrant said.

Delta tested the process last summer in Atlanta and Los Angeles and saw some reduction in boarding time, Durrant said.

Gary Leff, co-founder of frequent-flier website MilePoint, said the service will be the biggest help to passengers in the final boarding groups — the ones most likely to find the overhead bins full. Their bags will go in the cabin instead of being gate-checked as cargo.

"This has the potential to come across as a nice, high-end service," Leff said, "but I'm skeptical that it will go mainstream" because of labor costs.

Southwest Airlines wants to reduce complaints that families can't find seats together because flights are so crowded.

Unlike most airlines, Southwest does not offer assigned seats. Instead, passengers line up at the gate by group — first "A," then "B'' and finally "C'' — and pick their seat once they are on the plane. The system lets families board together after the "A'' group, but only with children up to 4. Some families pay extra for priority boarding to improve their odds.

Flight attendants often have to ask other passengers to move to accommodate older children or families that don't get to the gate on time. That usually works, said Teresa Laraba, a senior vice president who oversees customer service, but Southwest recently tested expanding family boarding to include children up to 6, 8 or 11.

"We've always tried to finesse it," Laraba said, but the test is designed to see "if there is a tweak that would improve the overall experience for everyone."

The airline is now surveying customers and expects to make a decision in a few weeks, she said.