Level streaming with landscape makes game very slow

Hi,
I am trying to make a procedural map using level streaming. I have a main persistent level containing the sky sphere, directional light, skylight. And seperate 6 levels containing a single landscape in it.
I tried to make a full map by randomly repeating the 6 levels with a grid size of 16x16.( 256 level instance). But the problem i am facing is after the map is loaded the game play is very slow and my frame rate is dropping below 10.

Where as a single landscape of same dimension (component size, section size etc) it is very smooth with frame rate above 70. Also if i put a high detail static mesh in a level and no landscape to make the world, still the frame rate will be above 70.

I have checked the stats and finding that gpu use is very high and is slowing down overall frame rate below 10. Inside gpu stat Basepass and Shadow depths are taking max time. I tried disabling the shadow setting for each of the 6 landscape but still no effect, even i made the landscape with minimum dimension that is section size 7x7, number of components 8x8 but still no effect.

I am clueless about what i am doing wrong here, i followed the same example as given in youtube to make a procedural map. Please let me know how to solve this.

Best regards
Atin