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Does removing overlaping geometry that is opacity masked improve performance ?

I am working on a freezing surface effect, my material moves the surface vertices of a tessellated plane so that they resemble spikes, and a sphere mask reveals or hides a part of the mesh.
The hidden part of the material shows up as red, but if I move my camera so that its parallel with the surface, the hidden part show as magenta.
The strange thing is that if I multiply my sphere mask with the vertex movement, so that the material surface is flat when hidden, the performance increases.
Is this behaviour normal ?