Big Apple bus riders may be better off walking.

The city’s slowest bus route — the M42 — slogs along 42nd Street between First and Twelfth avenues at a sloth-like 3.9 mph, a city comptroller’s analysis found.

On the heels of a recent report blasting New York City buses as the nation’s most sluggish, Comptroller Scott Stringer’s office has pinpointed the slowest and most chronically late routes.

Transit watchdogs say the findings are appalling.

“It’s frustrating that it’s faster to walk than take public transportation,” said Stephanie Burgos-Veras, an organizer at the grass-roots Riders Alliance. “The data proves that buses are failing New Yorkers.”

After the M42, the next slowest bus routes, all in Manhattan, are the M31 (4.1 mph), M57 and M66 (both 4.3 mph), and M50 (4.5 mph), the comptroller found. The average speed of all city buses last year was 7.4 mph; in Manhattan, it was 5.5 mph.

The analysis tagged the SBS15, which goes between East Harlem and South Ferry, with the worst on-time performance: 34 percent. Close behind are the M1 between Harlem and lower Manhattan (36 percent), the Q113 in Queens between Parsons and Seagirt (37 percent), the S86 in Staten Island between the St. George Ferry and Mill Road (38 percent) and the SBS60, between the Upper West Side and La Guardia Airport (38 percent).

“The slowest bus system isn’t in Los Angeles or Boston, or Philadelphia. It’s right here at home,” Stringer said in a statement. “If we’re going to be a true five-borough economy, we have to modernize our buses and connect routes to new job centers. It’s time for an overhaul.”

Burgos-Veras agrees. Her group has called on the MTA to redesign the bus routes to get riders to key destinations faster, and on Mayor de Blasio to add bus lanes, better enforce those that exist, and install new traffic-light technology to avoid frequent stopping.

Slow buses harm New Yorkers’ quality of life, she said.

“If they’re late to work, they lose money. It has a trickle-down effect on making the rider’s life difficult.”

MTA spokesman Jon Weinstein blamed the de Blasio administration for failing to tackle traffic congestion: “We need a real plan to deal with this intractable problem, better bus-lane enforcement and the political will from City Hall to actually improve bus speeds.”

The city claims it has boosted NYPD traffic and bus-lane enforcement, and says it will add 21 Select Bus routes over the next decade.

The comptroller plans to unveil today a new website where riders can look up data on each bus route — including average speed, reliability, number of turns, and distance between stops.