Fps jumps on a good PC (little fps) (Fortnite)

Hello everyone, after buying a new PC, there was a problem (only in fortnight), after starting the computer, I turn on the fortnight, the fps is excellent, about 300, then I fold up and open the browser, the fps drops to 100, the RAM is not loaded, the percentage is not hollow, vidyuha too holds up well, after closing, fps remains about 100 fps, only restarting the PC helps, but after starting the browser, there is little fps again, sometimes after several ALT + Tab fps can return to the previous 300, but after 2-3 seconds it immediately decreases to 100 , tried to check the game, did not reinstall the game.
PS That there is such a problem on direct 11, that on alpha (performance), the settings are minimal.
pc specs: ryzen 5 3500, 1060 3 gb, 8 gb Ram 3200 MHz.