Tessellation is definite NOT extremely expensive.
The way that tessellation is implemented on landscape materials is either bugged, broken, or the source of all evil, take your pick.
You can try exporting a single landscape tile out. Making it into a static mesh with all its child LODs, and placing the same material on it with tessellation.
Remove the landscape, replace everything with the custom meshes. No more FPS hit. (Also no more layers or way to modify, but you gain close to 40fps, even 60 at 4k.).
It doesnt even matter if your landscape has tessellation or not. Once your material.has tessellation activated you get a loss of fps. A drastic one at that. Even with the multiplier set to 0.
The current solution is to simply not use the landscape system (or the layer blend materials) as the whole system cause huge performance drops compared to static meshes.