Goals from Robin van Persie, Tom Cleverley and Wayne Rooney saw the Red Devils race to a three-goal lead within an hour, before substitute Fraizer Campbell headed a consolation for the Black Cats.
The win followed up last weekend's derby victory over title rivals Manchester City and restored United's six-point lead at the top.
Ferguson told Sky Sports: "For an hour it was fantastic football, some of the play was great - combination play, interchange play. The second goal epitomised that play, a great one-touch finish by Tom Cleverley (after a one-two with Michael Carrick).
"I don't know whether we've relaxed or Sunderland have picked up the baton towards the last 20 minutes but they had two or three chances then and did very well in that period.
"So I'm happy we've won the match. I think we should have scored more goals but you've got to give credit to Sunderland for keeping going."
The win was United's fifth league success on the bounce and Ferguson was pleased to keep up the pressure on their rivals at the top.
"The name of the game is to win games now, get the momentum going, get the consistency going and hopefully we keep going."
Another boost was the return of club captain Nemanja Vidic as a second-half substitute for his first appearance since September.
Ferguson added: "Well it will take him a bit of time (to get full match fitness) but that's a start. It's great to have him back.
"All the defenders are fit now, Rafa(el) had a little tight hamstring, we didn't risk him, but he'll be available for next week so that collection of defenders will definitely help us."