Hard to say what is causing this without a little more information.
What are the specs of your PC, and what exactly do you mean by maximum size settings of the landscape?
The last one most importantly, because as far as I am aware there is no maximum. Only maximum single tile size.
So by maximum size, I assume you mean “8129x8129”. Which if its at 100% scale will but just past 8km in X and Y.
I imagine the issue of lag is caused by the landscape size. Even on a beefy computer 8129x8129 can drag a bit. Have you considered using world composition so that the landscape can be loaded in in chunks?