Though there some apps around in the Google Play Store claiming to fix the lag or increase your phone's performance, those are mainly placebo apps providing no-till very little help!
Here are some tips on how to fix "lag".
1. Verify that your Android phone is compatible and meets the recommended system requirement of Free Fire.
2. Graphics: Slightly decrease the quality of the graphics in the configuration menu.
3. Update your Android system regularly or turn on auto-update in the system settings.
4. Clear the temp cache of your Android phone, clear saved data of apps and games you do not need via the app settings menu.
5. Check if all your games and apps are optimized for battery usage and performance via the system settings.
6. Regularly close games or apps running in the background as they might slow down the performance or cause lag.
7. Restart: If all of the above fails, restart the phone completely.