It’s difficult to create large landscapes because everything severely degrades performance.
I experience great terrain efficiency when using the following awesome plugins:
I don’t know if this fits your particular use case, but it might help. I guess you can use the plugins without random terrain generation, but with custom terrain. I don’t know which one though, as I use it for infinite procedural terrain generation.
These plugins are hard to use, but as far as I know, offer the best performance while using large terrain landscapes.