As a leftover effect of my odd mix of hand editing + region generation I have LARGE batches of landscape that is covered by rock/mountain/cliff that the player can't see that still has grass on it. How much is this draining performance? Is grass one of those more intensive things, or is it barely anything? Would it be worth it to go through every single mountain in my land and cover it with a grassless rock texture to get rid of the erraneous grass? Or would that not really buy me any extra performance?

Just wondering where I should prioritize doing this. Thanks!

Grass is a double hit on performance. On the GPU to draw the grass in the first place, on the CPU to Shadow it. In the past mods that have reduced the height of the grass had no affect on Frame Rates, the mods that reduced the detail of the grass would improve frame rates.

Since the player can't see this grass, could an LOD grass texture be used for those areas?

So basically I should definitely do this then?

It's simply behind/under rocks and stuff (The mountains) so I'll toggle the mountains then paint over it all with a blank rock texture to get rid of the grass. Sounds like that should really help performance.

This is true for everything.

The performance hit from grass can be lots of things. Make sure you don't use transparency AA for one. I believe Skyrim uses instancing for some grass (at least Boris, the author of ENB, believes so) so having tons of grass polys on screen is never the actual issue... until you add stuff like transparency AA. On the CPU side, it seems the Skyrim engine buckles under the weight of having thousands upon thousands of objects in its scene graph. The more objects, the harder the scene graph is to traverse, the longer it takes to update, etc. etc.

What would make sense is if grass meshes were made which consist of several clumps of grass. So if you had a grass NIF with 10 clumps of grass, instead of placing 100,000 grass objects around, you could maybe get away with 10,000. Of course this runs into the issue of looking repetitive.

Would it help to place occlusion objects under/inside of the mountains to occlude the hidden landscape? I have noticed that Bethesda designers placed occlusion inside of most exterior structures. I have assumed that was to prevent the player from rendering objects on the other side of the buildings. With that thought, won't you grass fade distance stop you from rendering most of the grass that is hidden by an entire mountain? I had assumed fade distance was why they didn't put occlusion objects inside of mountains - since you can't see anything but lod on the other side of the mountain anyway.