Orient turned in a stunning display after falling behind against the run of play when skipper Tommy Rowe edged Posh ahead in scruffy style in the 26th minute.
But class soon prevailed as three goals in the space of 20 minutes transformed the game.
A classy brace from hot-shot David Mooney sandwiched a stylish Dean Cox strike as Slade's side ran riot.
Mooney had thumped the crossbar with a dipping volley before firing Orient level on the turn seven minutes before the break.
Furious Posh chairman Darragh MacAnthony branded his team's performance in front of their biggest crowd of the season as "disgraceful' on his Twitter account.
Ferguson stopped short of that level of criticism and remains adamant Posh will play a full part in the title chase.
Ferguson said: "There is no doubt Orient were the better team on the day and full credit must go to them.
"You don't win the amount of games that they have done this season by fluke. I was happy it ended 3-1 because it could have been 6-1.
"They were better than us in every area of the pitch. They won every header and they won every tackle.
"In all truth we haven't performed to the required standard for the last few games and there is no doubt we need to do better.
"But no league title is won in November. I fully expect us to be right up there in May when it matters."