Mike Napoli can’t explain it. Perhaps he’s just a slow starter, although he pretty much punched a hole in that theory this season by having the best April of his life. Maybe he likes the summer heat. Or could it be that he thrives on the pressure of a pennant race, when the stakes are highest and the spotlight is blinding? Regardless, Napoli always has saved his best for last, as in the last half of the season. If the All-Star break is over, chances are Napoli is just getting started, which is only good news for the Red Sox given the bearded slugger’s six-week offensive snooze-fest. Suffice it to say, Napoli’s second half is off to another slamming start. Forget about his Wall-clearing three-run homer against Yankees ace CC Sabathia in the third inning last night at Fenway Park. Napoli ended a 4-hour, 46-minute marathon in the 11th by demolishing a two-strike pitch from reliever Adam Warren to straightaway center field to account for an 8-7 win in the finale of a three-game series. “I was looking to drive something,” Napoli said, referring to a pair of preceding swing-for-the-fences hacks that would have sparked a strong breeze in a desert. “Every time I’m up, I’m trying to drive a ball somewhere. I got a ball up and made good contact.” Napoli’s walkoff shot also came on a night when he struck out three times and grounded into a rally-killing double play in the eighth inning with a chance to spare the Red Sox from having to slog through another three innings and play nearly an hour past midnight. But it was vintage Napoli, who leads the league in strikeouts and the Sox in the longest homers hit this season. Until last night, he had gone deep only twice since June 1. But this is the second half of the season, when everything usually changes for Napoli.