Your landscape material is too complex, I would suggest you hard code some parameters, there are way too many parameters and this might affect performance. POM cost depends on the screen coverage. Since the landscape covers most of your screen space it will end up being slow to render. Try to split up your terrain into multiple tiles and make a simpler version of the material without POM to use in the middle ground and background. You never get to see the POM effect that far anyways.
If you want to debug the performance, I would try adding a switch to turn OFF and ON POM just to make sure that the issue isn’t coming from there.
Also note that the amount steps used in the POM Mfunction dramatically affects performance.
On the other hand POM isn’t really recommended for shading landscapes. POM cost will go up for each landscape layer added. IIRC POM is around 300 instructions and if you multiply that by the number of landscape layers you will realize that you will easily go over 3000 instructions for 5 layers (Normal and Albedo).
Use Tessellation when possible and POM for only some areas that need some details as a separate mesh or decal.