Your GPU and CPU are not running at higher usage because they dont need to. Most people use a normal 60Hz monitor, which only displays 60FPS in actual frames, though a higher FPS can appear smoother. If this is the case with you, then the fact that you are "only" getting 150FPS does not change anything. Unless you have a high refresh rate monitor, which you havent mentioned, then its not even worth your time or the "lost performance" to fix, since nothing would change anyway, other than the usage number for the PC going up. Even the LoL website on hardware requirements says that 90+ FPS is pro-tier, and doesnt bother to measure higher FPS on their charts. I do not personally play the game, but from what I do know, it is not a game where raw FPS can be the difference between winning and loosing, like an esports first person shooter title.
That being said, limits could be caused by the game architecture, or a limit that is in place, like an FPS limit, or a power limit.