I've been experienced some FPS drop with G-Sync activated when playing League of Legends. What happens is that the game starts with about 300/400 FPS and after some seconds, they drop to about only 30 FPS, or even less.
My solution was to disable G-Sync to that game on the Nvidia control panel and that was running normal.
This was happening in version 188.8.131.5286.
Now, Nvidia launches the version 184.108.40.20619 and they solved this issue. I can now play LoL with G-Sync ON and have no FPS drop. Just installed the update that GeForce Experience was asking for...
However, it is still necessary to block the FPS in game to 144, instead of default uncapped. If you let it in uncapped, above 300 fps it will do an image "error", appearing that the player "flashes" when you click to move it.
So, you need to update to latest driver, turn G-Sync ON and open the game. In the Video Configuration, blockyou max FPS to 144.