I need pretty detailed grass, I was using around 8k instances at the moment (but planes themselfs are kinda big, probably bit bigger than usually used, covering bit more space than expected). My issues is that everything was fine and fast before 4.6. I was waiting around 30 minutes at preview build quality and recently I managed to complete build with grass, but not sure how long it actually took. Result looks exactly same as in 4.5 but took significantly more time (I was able to build whole level in under minute and half in 4.5.