Two pet peeves triggered here.
First, don’t measure performance in “frames per second loss.” Going from 340 fps to 300 fps is very different from going from 42 fps to 2 fps. Measure frame times in milliseconds, because that is directly comparable.
Second, it’s very unlikely that the landscape is causing your GPU to bottleneck. I would bet money that the limitation is the CPU, not the GPU. I could be wrong, but this is what my experience tells me. Thus, you might want to run a variety of benchmarks (the standard ones for graphics/CPU bottleneck diagnosis) to figure out what the problem really is.