SysDiffuseshader, which only shows the diffuse map. It is the most lightweight shader that exists in the game that can display 3D.
If you're trying to get the most performance out of the game, then I would recommend only using this mod as a last resort. It completely destroys the visual look of the game along with causing issues such as mouth morphs not working in cutscenes, missing transparency on some textures, skyboxes not moving, and certain mods crashing the game.
We will not be fixing any issues that come from using this mod. You have been warned.