The client runs best at default window size: 1280x720.
You can find low spec mode by going to your League settings (the cog at the top right of the client) and checking the box next to 'Enable Low Spec Mode' on the first settings page. In return, you should see a performance increase. Low spec mode turns off several client animations and visual effects. If you're seeing lag or FPS drops in-game, please enable the Close Client During Game function at the top of your League settings, or check out our low FPS guide. Note: These steps are intended to improve client performance, not in-game performance. Below are some steps you should take if the updated League client is slow, laggy, or freezing up on your computer.