All three Ferrari-powered teams - Haas, Alfa Romeo and the Scuderia itself - have struggled for straight-line pace in 2020. Ferrari team principal Mattia Binotto confirmed after Sunday's Austrian Grand Prix there was a 0.7secs gap in this area.
Magnussen added: "All I see is we are slower on the straights this year compared to last year, having to drop more downforce than we did last year. That's all I know.
"I can't even say for sure that's the engine. I can just conclude that we're slower on the straights, and we're having to drop more downforce than last year."
Asked if the extra cooling being run to prevent a repeat of the brake overheating problems of a week ago was a factor in the loss of pace, Magnussen added: "It has had a little bit of an impact, but not a huge amount."
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