Posh struggled for a foothold in the game but were given a free opportunity to take the leaf in the 44th minute when Ephron Mason-Clark was fouled in the box.
Captain Harrison Burrows converted the spot-kick before Jadel Katongo and Kwame Poku stretched the lead. Nick Freeman scored a late consolation put Posh held on to keep well in the automatic promotion race.
They now sit just four points behind Derby in second with a game in hand and three behind Bolton in third, having also played a game less.
Especially in a season where goal difference could prove crucial, Ferguson has been pleased to see his side putting teams to bed in recent weeks.
He said: “We’re finding ways of winning games and we’ve managed to do that again with another three goals. We’ve overcome a tough opponent but we were nowhere near our quality.
“I can’t expect perfect all of the time though, what I need is that we find a way to win when we’re not good and tonight we were ruthless.
“It’s a reverse to what we’re used to and we’re being ruthless at the right time of the season.
“Malik has come on and influenced the game, all the subs did. It’s a really good win
“We made it hard for ourselves by not moving the ball into the areas we needed to but it’s an important one.
“I know the sort of team they are and even when we got the third, I knew that we would cause ourselves problems if we let them get a goal back at any time of the game and it proved that way.
“They scored one and were launching the ball into the box, they’re a hard team to play against. That’s why they are where they are
“Jed was our best player and was the best with the ball at his feet as well. He was outstanding. He’s used all his experience and was calm and composed.
“He made one excellent save just before it went 2-0. His calmness was so telling, I was really pleased with him.
