It appeared as though the League One table had been flipped on its head during the first half-hour at Fratton Park.
Relegation threatened Reading dominated the opening stages and forced Will Norris into action on a few occasions.
But they did not have anything to show for their efforts and once Paddy Lane had broken the deadlock, Pompey showed just why they are top of the table.
Marlon Pack doubled the advantage, with Callum Lang and Colby Bishop adding further goals before Charlie Savage grabbed a late Royals consolation.
And John Mousinho was delighted with the way his side overcame their early troubles to register such a comfortable victory.
The Blues boss said: “It felt like there were about 25-30 minutes of Reading being on top and they caused us a lot of problems.
“We had to adjust the way we pressed and were a lot better after we’d done that, getting the goal at just the right time.
“It became more open after the break because of the fact they were chasing and we managed to be clinical, taking advantage of our chances.
"We had to adjust the way we pressed and were a lot better after we’d done that, getting the goal at just the right time."
“We knew that we’d have to deal with their aggression and if we could pick up the second balls, then it’s about what we can do.
“It was about showing a bit of composure and there would be opportunities – I always felt there would be goals in the game.
“We were ruthless and some of the criticism of this team during the season is that we haven’t always been.
“That clinical side is something that we do have to develop, so it was pleasing to see it on this occasion.”
Leading marksman Bishop replaced Kusini Yengi up front, while Myles Peart-Harris was absent from the squad completely.
Mousinho explained: “He picked up a minor knock on Tuesday night and just felt his knee slightly when he tried to train on Friday.
“We’ll get an update on him on Monday morning, but it doesn’t appear to be anything too concerning.
“We’ve got two centre-forwards champing at the bit and we’re not weaker with either of them in the side. It’s a privilege to have that option.”