The problem is, that Rebirth has a huge amount of materials/ textures. Reworking all of them would take me several weeks and i am sadly way to busy to do that

The new shaders and improvements in the renderengine will open some possibilities further down the road.
EDIT: the shaders do not really affect performance. We need a bit more Vram to store the additional texture there, but aside of that they should be as fast as the old ones. What makes the renderer slow is for us more the amount of objects in a scene (drawcalls more or less)