Masked shader complexity

Can you explain why the hidden part of geometry is heavy to compute? Even if “Mask material only in early Z-pass” is checked.

Yea I agree, the PS cost should not be affected by the masked portion if you are using the “Mask material only in early Z-pass” option. Did you make sure to restart your editor and maybe invalidate+recompile all shaders? You might also want to ensure that your prepass is rendering everything including movable objects.