MobCaptains-v2.1.2.jar
Insanity-Shader-Universal-v1.650.zip

The particles of the mob captain are visible at a great distance, this certainly affects the FPS. I do not know if it is possible to disable their visibility, but if these particles were not displayed at such a huge distance, it would be better, because I saw it with shaders from an insane distance only thanks to the particle.

For that you'd need to modify the code, but you can do that.

1. Open the .jar file, this can be done with an unzipping tool or even by renaming it to .zip instead of .jar (remember to undo this in the end)
2. Navigate to data/ps-mob/particles/
3. In this folder you'll find some .mcfunction files, open those with any text editor, the pre installed windows editor works
4. In each of those files, you will find lines starting with particle and they end with force, change that force to normal and save

Edit: I messed up what to change, 4. should now be correct