Far Shadows on Tiled Landscape Performance

Hi there, I have a 9km by 9km tiled landscape. When I enable far shadow cascades on my directional light, my framerate drops from 200+ down to 18fps. I do have VXGI integrated with the build, but it only works within a small distance from the player. Has anyone else run in to this issue? For the record, I am using an Intel i7 8700k, 32GB of RAM and an Nvidia GTX 1080ti.