I am not sure how UE4 decides to render a static mesh and when?
but here is my problem:
my scene has approximately 180k tris in the screenshots, when the light intensity is is above 0.5 my total tri count jumps over 5 million.and causes lag.
where does it come from? (the landscape is very large and I assume this is where the issue originates from, but doesnt UE4 terrain only render the visible geometry ?
please explain if this is somewhat common sense 
Dynamic Light on
Dynamic Light off
The triangle increase comes from having to render everything relevant into the shadowmap, for every cascade. The geometry might not be visible to the camera, but this geometry might still contribute to the shadow, hence the increase. Try adjusting shadow settings, including distance and number of cascade splits, aswell as doing more aggressive LOD transition on your landscape. Also consider using world composition with level LODs, if your landscape is indeed that large.
Hi Deathrey,
You pointed me in the right direction, there are still a lot of issues but at least now I know where to focus.
Thank You
