The match saw skipper Raphinha hit a hat-trick and also Gavi's return from long-term injury.
Asked if it had been a perfect day, Flick said afterwards: "No. Gavi's return has been fantastic and the five goals too, but the pace of the game has slowed down.
"I didn't like some of the pressure, related to winning the ball back... There are things to improve. I'm happy for the three points, of course, and because Gavi, Raphinha, Xavi Espart... did a fantastic job.
"There are many things going well, but there are situations on the pitch that aren't fantastic. It's five goals and three points. But at the beginning, we didn't play with the conviction I want to see. We have many games left, but we need to play faster. We need to focus; Newcastle is a very good team."
On Joao Cancelo, Flick added: "He still needs time to adapt, he lacked match fitness... He has technique and won two penalties. Bringing in Cancelo was the right choice, but he needs a little more time to adapt."
And on Gavi's return, he also said: "It's fantastic that he's back, the medical team has done a fantastic job. It's a step in the right direction. We don't know if he'll play on Wednesday or Sunday... but the opportunity arose today.
"He has good technique, good passing, good one-on-one skills... we're all happy that he's back."