Remember that slow start by the Mets when folks couldn’t help but wonder if they could back up that magical World Series run last season? Well, forget it. The Mets are who we thought they were — and then some.

Not only do they have one of the best pitching rotations in baseball, but their revamped lineup is proving they have the bats to match. The Giants figured to pose a major test, but the Mets hit them between the eyes with a 12-run third inning that was the greatest in club history on their way to a 13-1 rout Friday night at Citi Field to extend their winning streak to seven games.

Giants starter Jake Peavy (1-2) struggled with his control early, and when he walked Mets leadoff man Curtis Granderson and then David Wright to start the bottom of the third inning, it was as though he lit a fuse to a powder keg that inevitably exploded in the Giants’ faces.

The Mets’ half of the third inning ultimately took 39 minutes, 47 seconds to complete and included 66 combined pitches from Peavy and reliever Mike Broadway, each of whom gave up six earned runs. The Mets sent 15 batters to the plate in the inning, and the only one of the first 13 who made an out was pitcher Steven Matz (3-1), who struck out trying to bunt.

Everyone in the Mets lineup but Matz scored in the third inning, including two runs each by the top four in the lineup. Yoenis Cespedes, who was making his first start since having a thigh bruise drained, delivered a two-run single off of Peavy, followed by a grand slam off of Broadway that completed scoring in the inning and gave him six RBIs before Lucas Duda and Neil Walker made the final two outs.

After the opening two walks of the third, Michael Conforto stroked a single to left-center for the game’s first run. Cespedes’ single to center pushed Wright and Conforto across for a three-run lead before Peavy walked Duda.

That brought up the hot-hitting Walker, who crushed a Peavy delivery that looked like it might go out for Walker’s 10th home run, but it stayed in the park for a run-scoring double and a 4-0 lead. That was the first extra-base hit of the season by Walker that didn’t leave the park, and it ended Peavy’s night.

Broadway came on and immediately got lit up by Asdrubal Cabrera, who drove a two-run double to left-center for a 6-0 lead. Catcher Kevin Plawecki then walked before Matz made the first out.

That brought up Granderson for the second time in the inning, and he drove a ball to the warning track in right-center that was dropped by Hunter Pence but credited as an RBI single, scoring Cabrera for a 7-0 lead. Wright singled to load the bases, and Conforto singled to right to push across the eighth run and leave the bases loaded.

Cespedes is the last man Broadway wanted to see in that situation, but he had to pitch to him. The Mets’ cleanup hitter lined a first-pitch slider just over the wall in the leftfield corner to make it 12-0. That was the end of the carnage.

After his long rest, Matz returned to the mound and proceeded to complete six scoreless innings in which he allowed seven hits, walked three and struck out four. Matz’s 110-pitch effort hardly was dominant, but it didn’t have to be.

He was replaced by reliever Jerry Blevins, who yielded a leadoff homer in the seventh to the Giants’ Angel Pagan. The Mets tacked on a 13th run in the bottom of the seventh on an RBI single by Juan Lagares.