On some routes, the last-place airline is over 20% slower on average than the speediest. But no traveler gets the average flight, and the slower airlines tend to have more variability in their performance. Catch that carrier on a bad day and the gap can be much wider.

In some cases, the average difference in flight times is just a few minutes, which may not seem like a big deal from a flier’s perspective. But to an airline, an average of just 2 minutes slower across its fleet makes a significant difference in costs and could require more planes and crew to cover the same schedule. That comes back to the traveler in reduced amenities, more nickel-and-diming on fees and higher ticket prices.

For the accompanying list, The Fastest Airlines In The U.S., Forbes and AERA evaluated the big carriers’ performance separate from the regional airlines that they hire to provide service in their name.

This article evaluates the major airlines and their contractors as one, useful for travelers interested in the best options on a particular route. When booking a ticket, many may not notice the fine print that their American flight is operated by Compass Airlines.

(Note: Forbes skipped the ninth-biggest route, Honolulu-Kahului, because of lack of competition. In 2018, the only other carrier on the route than Hawaiian Airlines was a small plane operator called Mokulele that doesn’t transmit flight information through the ACARS system, whose data partner AERA used for this analysis.)

This is the most heavily flown airport pair in the U.S., with about 19,000 flights in either direction in 2018. Southwest was the fastest airline both ways, with average actual flight times 8 minutes quicker than last-place United going from Los Angeles to San Francisco and 10 minutes faster going the other way.

Flights on this route average 9 minutes slower going from San Francisco to L.A., in part because departing flights must loop north and west before turning south to L.A. (Note: Virgin America may be no more, but Alaska continued to operate flights under that brand through April 2018.)

SFO-LAX may be the single busiest airport pair, but New York to Chicago is the busiest air route in the U.S. when you total up all the traffic between the multiple airports in the two metro areas.

United performed significantly better between its main hub of O’Hare and LaGuardia than Delta, while American, which also has a large operation at O’Hare, was fastest flying into the Windy City. Spirit was the speed king into LaGuardia and only a few minutes back going the other way, but with just two flights a day in each direction, it may be avoiding some of the slowest times to fly.

On average it’s much slower flying from LaGuardia to O’Hare than the other way, with an extremely wide variability in airlines’ performance, as measured by the route index standard deviation. That’s not just because planes are flying into the jet stream: All planes headed into O’Hare from the northeast line up at a single arrivals fix. “It’s all nose to tail from Syracuse westbound,” says Michael Baiada, a former United pilot and industry consultant.

Southwest was 8 minutes faster on average than last-place Alaska from Las Vegas to LAX and 10 minutes faster going the other way. Part of the problem for Alaska is that Delta and Southwest have terminal locations closer to the runway at LAX that allow them to taxi out 6 minutes quicker. This is one of the most efficient and least variable of the 21 biggest routes.

JetBlue and Delta look a bit better than Alaska, while American lags. Flying west into the wind, there can be a lot of variation in arrival times from JFK to Los Angeles. It’s not unusual for flights to come into LAX 45 minutes early in favorable weather, says airline consultant Bob Mann. This route carried about 5.9% of airline traffic in the U.S. in 2018, but it accounted for 10.9% of the delay minutes over minimum block, by AERA’s calculations, the most of any city pair.

Portland-Seattle is a big route for Alaska, with roughly 8,000 flights in either direction, mostly on its regional subsidiary Horizon’s planes. It’s best to pick a Horizon plane rather than one of the parent company’s—Horizon’s fleet of Q400 turboprops are faster on short-haul flights. That’s partly because of the aircraft’s lower cruising altitude, which it reaches faster, meaning it spends less time ascending and descending.

Delta is the clear winner both ways, averaging 19 minutes faster than United and 8 minutes better than Alaska into Seattle, where Delta has challenged Alaska for market share at its home base over the past few years. Going the other way, Delta is 7 minutes faster than Alaska and 14 minutes faster than United. United has contracted heavily for service on this route with SkyWest, which flies regional jets that are smaller and slower than mainline planes.

JetBlue launched service on this busy shuttle route in 2016, challenging American and Delta and sparking a reduction in ticket prices. Last year JetBlue was the fastest both ways. All three airlines serve this route with regional jets. Efficiency is higher for flights from Boston to LaGuardia than on a number of other routes into the Queens airport because flights coming from the north aren’t impacted as much by JFK and Newark airports, says Baiada.

United is top of the table while Alaska brings up the rear between its two main hubs, but there isn’t a lot of separation between airlines on this route. Delta put significantly more pad in its schedule on this route than its competitors.

Southwest beat Delta going in and out of its own hub on this route, which is among the most efficient in the top 21.

Once again Southwest is the fastest both ways, beating out Delta, though not by much. Flight times average 21 minutes slower from LaGuardia to Atlanta than the other way around, with higher variability. A big contributor to that is departure delays at LaGuardia, where taxi out times averaged 26.3 minutes last year, about 10 minutes longer than in Atlanta.

Southwest looks much better than its competitors on this short route connecting Tinseltown to Silicon Valley, with a route efficiency index half that of second-place Delta.

A strong showing by United out of its Denver hub, where it’s been beefing up service, but Southwest was still even with it into LAX and 6 minutes faster on average into the Mile High City. American, as with most of the biggest routes on which it flies, was at the rear, in keeping with its ninth-place showing out of the ten major airlines in our efficiency index average.

Delta led by a nose on a route with not much separation between the average actual block times.

Southwest is the fastest by a good margin, 13 minutes ahead of American into LAX and 9 minutes ahead into Phoenix. Phoenix is a big center for Southwest, but it’s also a hub for American, which usually confers advantages.

Delta offers the best round-trip performance into and out of its Atlanta hub, and American and United’s hubs in Chicago. Delta, which ranks fourth in our efficiency index, is helped by its stronghold at Hartsfield-Jackson Atlanta, one of the most efficient major airports in the United States. Elsewhere Delta faces the same complexity that United and American do in pushing large numbers of planes into congested airports, but Baiada say Delta does a better job at managing that complexity, and the average flight times on this route are a portrait in miniature. “That tells you that Delta manages a better hub than American or United,” says Baiada.

United was first and Spirit second flying between two of the most inefficient airports in the country. This route is the second worst behind JFK-LAX in terms of route efficiency: 25.2 into LAX, 23.8 into O’Hare.

The route between Washington State’s largest cities is the 18th most heavily traveled in the U.S., with about 15,800 total flights both ways last year. Alaska operates this route relatively efficiently, with about 65% of its flights served by its regional arm Horizon Air.

Once again Southwest leads both ways, with United, which uses regional jets heavily on this route, in the back.

Spirit Airlines ranks first, but this is a relatively strong showing by American out of its main hub of Dallas-Fort Worth on a long route with a fair amount of variability in performance.

JetBlue leads on this shuttle route. Average scheduled flight times on the Boston to Washington leg have ballooned past 100 minutes from 79 in 1986, and to over 90 going the other way from 74. In the last three years, Delta has increased its total amount of flights at Boston by about 30%, says airline consultant Bob Mann, challenging JetBlue for corporate customers and contributing to delays.