Brennan Johnson, Dejan Kulusevski and Dominic Solanke struck in the 3-0 win, with Bruno Fernandes sent off for United before halftime.
Postecoglou said: "I thought it was an outstanding performance. We started the game well, scored a great goal and probably should have had two or three. The pressure then overwhelms them and they get the red card. After that I thought we controlled the game well. Just an outstanding effort from all the lads.
"I don't know about frustrated (about not scoring more) but I am only human. More for the players themselves. We played some unbelievable football and you want them to get the rewards."
On the red card, he continued: "I think it was a consequence of how we were going in the game, just putting enormous pressure on them. The amount of chances that we were creating, we weren't letting them get out of their half. It was a strong all-round performance.
"Every game you always want to start well but it doesn't always happen. I told the boys that in the last two games we conceded in the first 20 seconds against Brentford and then had a man sent off in the first eight minutes, so it is about dealing with that you have out there.
"That is our fourth win in row and we have won them in different ways, but always stayed true to our principals and the way we want to play football."
On building momentum, he added: "It helps. I think our performances in the league have been strong all year but we haven't had that cutting edge. Our performances have been good but when you get the wins too that is important, you want the players to think they are getting the rewards for their efforts."