1: That shader trick slows down my laptop + A Desktop computer we have. (I found this out by using a GLSL shader mod that has waving wheat/grass) :/ And dynamic shadows that only use the sun to cast. And man, does it run slow.
Unless you try all of the other options I added then you have no basis for making that statement. Grass is different than other materials in more than just a free animation. If the grass animation is slowing you down then so are the clouds.
Anyway, the tests will say for sure but you have to run all of them and I will be able to tell where the slow down comes from.