Johnson scored 10 of his 25 points in the first 5½ minutes, and the 19th-ranked Boilermakers cruised to a 77-57 victory on Saturday.
Johnson was much more assertive than he was against Valparaiso on Tuesday, when he scored 13 points, but committed six turnovers and struggled against double teams. He made sharper decisions Saturday, often scoring with a strong move or passing to a teammate before a second defender could reach him.
"Just slow down," Johnson said. "Just be patient, look for the open man and make the right play."
Purdue coach Matt Painter said he thought Johnson improved against North Florida.
"I thought he made some shots early to help his confidence," Painter said. "He didn't allow them to double him quick. I just think getting the ball to him and establishing him early in the game really helped us."
E'Twaun Moore scored 21 points for Purdue (9-1), which won its fourth straight. Johnson and Moore combined to shoot 17 for 26 from the field.
Lewis Jackson, a 5-foot-9 point guard, had five points, eight assists and six rebounds. He has tied his career high for rebounds the past two games.
"Most people are worried about JaJuan and E'Twaun, and they block them out and don't think I'm going to go for it," Jackson said.
The Boilermakers shot 56 percent from the field to offset their 19 turnovers. Painter was unhappy with the mistakes.
"I just think we had some stretches where we turned the basketball over," he said. "We tried our best to get a lot of guys in there, and at times when you are trying to get guys some experience, it supersedes the mix of guys you have in there."
Jerron Granberry scored 14 points and Brad Haugabrook added 12 for North Florida (4-5), which committed 21 turnovers.
North Florida coach Matthew Driscoll said Purdue has one of the best defensive teams in the nation.
"They know when to bump and when to put hands on you," he said. "They're ability to guard the ball without fouling."
The Ospreys shot 52 percent in the second half, and each team scored 36 points after the break, but Purdue tinkered with lineups for most of the second half.
North Florida hasn't fared well against ranked teams this season. The Ospreys were routed 95-49 at No. 5 Pittsburgh on Nov. 13 and 96-58 at No. 15 Missouri on Nov. 20. They play at No. 25 Texas on Tuesday.
Johnson completed a three-point play after a spin and dunk with just over seven minutes left in the first half to give the Boilermakers a 35-14 lead.
Two 15-foot jumpers by freshman Travis Carroll gave Purdue a 39-19 edge. Purdue shot 63 percent in the first half and led 41-21 at the break. Johnson had 15 points in 13 minutes.
The Ospreys hung tough early in the second half before Moore got going. He made a 3-pointer, then got a steal and ran full speed into a 1-on-2 fast break. He took contact from one North Florida defender, then double clutched at the waist and laid the ball in while being fouled by the other. He converted the three-point play to give the Boilermakers a 57-31 lead with 12:20 remaining.
North Florida made a run to cut the deficit to 66-51 with just more than five minutes to play, but the Boilermakers quickly regained control.
Johnson said his performance was a result of solid team play.
"It was just being unselfish early on," he said. "A lot of those shots that we made were assisted shots, and that always helps. We had good spacing, and the offense was just flowing pretty well."