I’ve just noticed that if I include super high-quality materials to the landscape, it will increase the lag very much, and the only way to get it optimized is to disable tesellation which will remove the realistic looking texture.
But I don’t want other users to complain about the lack of optimization because of the insane graphics. So I wandered if is there a possibility to change the material from a landscape when clicking a option button or any button from a widget?
material quality switches
docs: Scalability Reference | Unreal Engine Documentation
So if I add a quality node in the material blueprint, then it will change the material whenever I choose low graphics or high graphics? And by that, I mean that I can change the material ingame?
Yeah the quality node is tied into graphics quality level and will change when the user selects different levels of graphics
Thank you soo much, my game shall now be played better. 
Topic solved 