Game development and game playing lagging problems

Dear to whom it may concern;
When I try to move around in my world when I am editing I am experiencing decent amount of lag. And when I play as my character I also experience some lag. I do have a basic landscape that is as big as it can go without filling the world with descent resolution. So is the lag because my world is to big? If so how can I fix the lagging problem and still be able to improve on my landscape. What in unreal engine 4 takes up the most computing power? Is it the physics box, the post processing volume, the lighting, the environment, or other?

What are your PC specs?

For large landscapes, the number of triangles and the complexity of the material shader are usually most important.
That being said: You give us nothing to reason about.
What’s your computer CPU? How much RAM do you have? What graphics card are you using? When you say “lag” what do you mean; do you have specific measurements?
Does it matter how big/small you make the window? Does it change if you make the material just a single texture of size 8x8 pixels?