With an eye on the predicted wet conditions, McLaren went for a higher level of downforce than its rivals with both cars, a move that paid off in the sprint event, where Oscar Piastri briefly led and finished second, with Norris sixth.
Nevertheless, the Englishman says that given more time the team would probably have taken downforce off the car.
"I think we're just not in the optimal range of downforce, I think, we went for a bit too high," he said when asked by Motorsport.com about his performance in the sprint.
"And that's costing us probably close to six, seven, eight-tenths, just on the straights alone.
"Then you add clipping and the battery harvest, it can easily be up to almost a second in the straights during a race. But obviously, we get a decent amount back in the corners.
"So there's always this little to-and-fro, but maybe just not in the optimal place, which is I guess the penalty of just having FP1 and qualifying on Friday, that we just weren't able to set the car in the right window.
"We're losing more than what we're getting at the end of the day, and we're paying the price for it."
Especially in dry conditions, a lack of straightline speed will make it harder to defend on the long run up the straight.
He added: "It's going to be a struggle at points. We know it, we just have to deal with it and do the best we can with what we got."
McLaren team boss Andrea Stella admitted that while higher downforce helped in the wet sprint event, his drivers will have to work hard to defend on Sunday.
"It has definitely helped our performance today by having a bit more downforce, which comes at the cost of drag through the rear wing," said the Italian.
"So in these conditions, we have seen already in qualifying it gives quite a lot of strength in the second sector, and also allows you to gain a little bit through Eau Rouge as well.
"So this mitigates the loss of speed from corner one to corner five, let's say. So definitely helping in today's conditions.
"And at the same time, it leaves us vulnerable tomorrow in some situations like after the start, and safety car restarts. So we are aware of that, we'll try to mitigate somehow, but we know there's a point which could be a problem tomorrow."
Stella explained that one reason that the team went for high downforce and didn't split the downforce level of the cars, was that it is more familiar with the upgraded MCL60 at higher levels, and hasn't yet fully optimised it in a lower drag configuration.
"One thing that we haven't attacked yet is actually the car at low drag level," he said. "So there wasn't much efficiency to gain by going on a small rear wing, let's say. And we decided to stay at the point in which the car is at the moment most efficient.
"So there's not only a tactical choice based on you want to be quick in the corners, or you accept to be slow on the straights. It's more like what actually the car has to offer as a function of the rear wing level. And at the moment, this is where the car performs the best.
"This is certainly something we plan to address for the future because then it creates a situation where the car is not necessarily very easy to race, especially as the conditions get dry and Eau Rouge is flat, and then you are exposed to other cars coming past."
