Reducing game lag in Unreal Engine

I’m currently making a game with UE4. When I test played it, the game was so laggy! I searched on YouTube and looked it up as well, but nothing helps, really.

My question is, how do I reduce lag in UE4? I have seen Improve Perfomance in Unreal Engine 4 but you have to reduce the graphics, then the lag will improve. There’s also videos on ‘performance optimization’, but again, that doesn’t help as much either.

How do I achieve no game lag without having to reduce my graphics for this?