A large number of Mac systems utilize Automatic Graphics Switching, which is enabled by default to allow your computer to automatically switch to the best graphics system for the applications running on your computer. On laptops, this is often enabled to maximize battery life.
However, using this feature may lead to FPS issues and general performance problems while running League of Legends. Disabling this option will use the higher-performance discrete graphics processor at all times. On laptops, this will decrease your battery life at the gain of higher performance while in game.
If you are having issues with FPS or general performance problems with League of Legends, disabling this feature is a good place to start.
Disabling Automatic Graphics Switching:
1. Open System Preferences
2. Click "Energy Saver"
3. Uncheck the "Automatic graphics switching" option at the top of the Energy Saver dialog.
4. Click the "Lock" icon at the bottom left of the dialog to prevent further changes.
5. Reboot your system.
It is important to ensure that you have installed all available updates for your system. You can find more information on how to run Software Update, here.
Important note for laptop users:
When running off of battery power only and the battery charge is low, you may notice significant loss of FPS. This can be remedied by plugging in the power adapter for your laptop. When using the discrete graphics processor and playing with higher quality settings, you may find that your battery will drain quite quickly. When possible, it is best to have the machine plugged into a power source while playing League of Legends.