I once ran into an issue, where landscape grass was being rendered 4 times for each shadow cascade. That was around 4.14. I believe it was project specific, since I was not able to reproduce it in a clean project with stock assets. The issue resolved on its own after deleting and re-importing landscape and never re-occurred in newer versions so I believe it has not much to do with this question, but might be worth mentioning.
Overall, high shadow rendering cost seems to be caused by sub-optimal landscape size/ shadow cascade settings. Smaller components, coupled with increased shadow distance and sharper cascade transitions should help.