My games are almost never laggy since I follow this dev forum post.
That might not be useful in this case since you said you have optimized every code in your game so perhaps check every detail in game and check for extra parts you can remove and resize other ones.
Try not to use extra ScreenGui since that effects too.
Firing many remotes at same time would cause lag aswell.
Instancing would be one of the best lag reductions I use.