Hello wmask,
I do see what you are describing, and it seems you do have a fix for your issue. Since grass is so dense you generally don’t want to even have shadows turned on for that amount of foliage. The warning message is correct as well, when you set each foliage instance to receive dynamic shadows that will take a lot of processing away from the areas where you will need it and become inefficient. I suggest turning off the dynamic shadows for those specific instances and you can turn on static shadows.
When it comes to larger objects like trees that sway, or even large rocks you can turn these shadows on and if their spacing is appropriate the shadows and processing required to avoid the error message should be fine. I have posted a few images to show you how I set up a little scene I worked on a while ago using the foliage tool and optimizing the shadows. Let me know if this helps you with your issue!
Shows that these objects are foliage
Shadows per instances of trees
For the Bushes in the scene I have unchecked the dynamic shadows and checked the static shadows. This provides a good compromise.