Does your laptop meet the Recommended System Requirements for the game? If not, then getting low frame rates is to be expected. Keep in mind that game updates & improvements can increase the recommended system requirements over time.
To maximize system performance make sure to quit all open apps and login items. Minimize the apps & drivers which run at system startup as well. Uninstall any anti-virus apps, cleaning apps, or third party security software since these are not needed on a Mac and usually cause more problems than they solve plus they greatly impact system performance. Third party proprietary NTFS drivers for any external drives are also a source of performance issues as users should only have macOS manage external drives.
Lower the graphics settings to a lower resolution and turn off some of the graphics options such as shadows, anti-aliasing, etc.
If your laptop uses a hard drive, then consider upgrading it to an SSD for better performance. If your memory is upgradeable, then you may want to add some more memory in order to meet the recommended system requirements for the game.
Check the League of Legends forums to see if this is a common issue on macOS. You may also want to contact the developers or publishers of the game for assistance.
FYI, if the fans are running at their highest speed and the CPU temperatures go over about 100C, then the CPU will become throttled down usually to about 800MHz. Apple laptops are not really made for gaming.
