I want to report this as a bug, see my answer:
If Stationary lights are in the near of a landscape which use the fooliage grass tool they interact with the landscape with big performance cost even the landscape isn’t in the radius of the light. Settting all lights in a dungeon to dynamic isn’t a good solution because they cost more performance as stationary lights (if the bug wouldn’t be).
Can you reproduce this in a blank project with sample test assets and provide the project here?
As I mentioned in the other post, a lot of things come down to profiling and optimization, which you did, but you have a very specific setup that would require too much setup on my end only to not see the same results. I’ll leave it up to you to provide the sample in a new project that recreates the issue. From there I’ll take a look and see if anything looks out of the ordinary.