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 — the greatest inning in club history — on their way to a 13-1 rout Friday night at Citi Field that extended their winning streak to seven games.

In his first start since having a thigh bruise drained, cleanup hitter Yoenis Cespedes again provided the thunder for such an explosive inning with a two-run single and a grand slam. That gave Cespedes two franchise records: six RBIs in one inning and nine consecutive games with an extra-base hit.

Cespedes said he’s not trying to live up to the fans’ expectations based on last season, but he added, “I feel at home.”

The Mets’ half of the third inning took 39 minutes, 47 seconds and included 66 combined pitches from Giants starter Jake Peavy (1-2) 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.

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“It’s the first time I ever had one like that,” Mets manager Terry Collins said. “There were some great at-bats . . . It seemed like every hit went in the gap.”

Peavy walked leadoff man Curtis Granderson and David Wright to start the inning. Michael Conforto stroked a double to left-center for the first run and Cespedes’ single to center pushed Wright and Conforto across for a three-run lead.

Peavy walked Lucas Duda, bringing up hot-hitting Neil Walker, who crushed a shot that looked as if it might be his 10th home run, but it stayed in the park for a run-scoring double and a 4-0 lead. The first extra-base hit of the season by Walker that didn’t leave the park ended Peavy’s night.

Broadway immediately got lit up by Asdrubal Cabrera, who drove a two-run double to left-center for a 6-0 lead. Granderson added an RBI single before Wright singled to load the bases for Conforto, who singled for an 8-0 lead, leaving the bases loaded for Cespedes.

In the Mets’ previous game Wednesday, Cespedes delivered a pinch-hit three-run homer to tie a game they eventually won. This time he lined a first-pitch slider just over the wall in the leftfield corner to make it 12-0.

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“What happened the other day I wouldn’t imagine would happen today,” Cespedes said. “It was the last thing on my mind. When I hit it, I knew it was gone, but when I went to home plate, I was just looking to make contact.”

After his long rest, Matz completed six scoreless innings in which he allowed seven hits, walked three and struck out four. His 110-pitch effort hardly was dominant, but it didn’t have to be now that the Mets have a batting order that gives the starters greater room for error.

Collins said the decision to sign Cespedes to a free-agent contract “was a huge move for us. It gave us a legit bat in the middle of the lineup. I like our lineup, and our starting pitching is starting to get on a roll . . . We’re going to enjoy the ride right now.”