Hiya! I just replied to your email as well, but just in case anyone else is looking to optimize:
- Update the asset (just to make sure, I improve something every couple of months).
- Disable any features you’re not using in the preset. This is usually the first checkbox in a group.
- Delete any blueprint paths you’re not using. This may or may not have any effect as the switche parameters in step 2 recompile the shader without those code branches at all.
- Adjust your in-game quality settings. Depending on your cel-shading settings you may not need the highest level AA, shadows, etc.
There’s one more thing you can do that might help. As you’ve said you’re doing cel shading, if you are also using cel lighting (or flattening out the lighting completely):
- Inside your asset materials and set the shading model to “unlit”.
- Attach your base colour input to the emissive channel instead (otherwise objects will appear black)
- Change the cel shading preset “Flatten amount” to 0.0 (otherwise objects will STILL appear black)
- Enable cel lighting
This bypasses unnecessary lighting passes by the engine for that material, since you’re not using them anyway. The lighting renderer uses a stencil to avoid processing unlit pixels so this is a good optimisation if you do it to most of your assets. The result will look like this:
If you get a black object then you need to check step 2, I often forget it. 
It also looks like not-unshading-the-sky has stopped working AND it looks like posterization might actually be useful on cel lighting. Another update coming soon I guess!