The mod author recommends iMinGrassSize=70 but I can't remember if I had FPS drops or just didn't like the coverage, so I lowered the setting (by increasing the number) and found sweet spot at 80 for myself. My Games/Skyrim Special Edition/Skyrim.ini I have used Verdant Grass since OG Skyrim and it has been utterly flawless, huge fields of grass, very dense and no FPS impact at all.
ini description pertaining to grass and for others to use these settings. Note that I have an i7 4790K (stock) and GTX 970 but I know many have commented that it's plummeted their FPS in half, and it really shouldn't be doing that.
I've just tested Verdant Grass for Special Edition.
