this might help improve performance (though probably the effect will be much smaller than you think), but it doesn't explain the comparative weakness of american PISA results.
Because: Finland? We have 2 month summer vacation in elementary school (189 work days/year). The school days seem to begin around 8.30 am. And we were long time the best country in PISA rankings (currently ranked 3nd after a city state of Shanghai and South Korea).
What many people who try to model the success of Finnish education system fail to see, is that Finnish society is highly equal. This is true between genders and especially between individuals from different social status and background. We work hard to give high quality education for everybody, no matter the cost.
This is the reason why sometimes I wonder how the projects that aim to export the Finnish school model to the countries like Saudi-Arabia can succeed, if the profound problems with the inequality deep in respective societies is not addressed first.
The PISA measures averages and it is in cutting the lower performing tail of the distribution where Finland excels.
Seriously ... summer holidays are pretty universal; it's not like countries generally considered to have very good educational systems don't have them.
China, Korea, Japan, all have 6-8 weeks of summer holiday; the U.S. seems to have somewhat more on average (of course in all of these countries there's some local variation), maybe 8-10 weeks, but that doesn't seem enough of a difference to have any dramatic effect.
The U.S.'s problems with education are cultural: American culture does not value education. School scheduling is mostly irrelevant.