cell-shading is less taxing on GPU, so changing it would result in performance downgrade.
This is misinformation. Celshading is a post-processing shader. It actually takes more GPU power to do cell shading than not to do it.
Cel-shading is a relatively cheap (aka fast) post processing shader, and other good looking alternatives do tend to be more taxing on the hardware. However OP wants what is already in the game, just without that specific post-processing step (as demonstrated by the glitch where the post-processing step is not correctly applied).
Thus what OP wants is faster, and less taxing on the GPU than having cel shading enabled.