With larger the visibility drops the performance because the CPU limited.
All settings on Ultra:
Visibility: up to 400m at 40FPS, CPU 30% GPU 99%
Visibility: up to 800m at 37FPS, CPU 30% GPU 73%
Visibility: up to 2000m at 33FPS, CPU 30% GPU 51%
Visibility: 12000m up in 9FPS, CPU 30% GPU 20%
What can help?