UE4 - The first time I test a build, starts with low FPS, why?

Whenever I do the first shipping build, when it finishes I restart the computer and start the game, but for some reason, that I don’t understand, the first build always starts with a low FPS of 40 to 45 and it starts overheating fast, BUT… If I close the game and star it again 2° time, now FPS is 60 to 90 and heat decreases, why?