But the 2018 car is also not fully up to speed -- notably on the straights.
"There is nothing serious or horrible in terms of the design," team boss Eric Boullier is quoted by Marca.
"But we have to be faster in the corners and on the straights."
The Spanish newspaper said McLaren may therefore be considering a 'plan B' -- like a new chassis for next month's Spanish grand prix.
"The two drivers think the same about the good balance of the car, and when we make a setting change, it reacts properly," Boullier added.
"The car is matching the objectives we set, so it's possible that the objectives were not right," he said.