When an application is open in fullscreen mode, Windows grants it full control of the screen's output. Assuming a game is optimized for the system and display being used, fullscreen mode has the potential to boost performance when compared to borderless windowed mode.

Why does fullscreen run better?

General: Games in Fullscreen have better Performance, just because the explorer.exe of Windows can take a break. In window mode, it has to render the game and averything else you have open. But, if it is fullscreen, it renders everything from your desktop when you shift there.

Is it better to play league in fullscreen or borderless?

Borderless is just more convenient because it makes it easy to alt tab out. Fullscreen is better for games like CS:GO because it has less input lag and framerates are usually better. When a program is running full screen your desktop isnt being rendered in the background by your os.