I guess that depends on the amount of Vertices/ triangles the N-Gon exists of (for rendering everything is being triangulated).
If the N-Gon has 4 Vertices, than it will of course be faster to render than a Quad with transparency 😉
If the N-Gon consists of 1 million Vertices, than that will render a lot slower than a Quad with transparency.
There must therefore be an amount of Verts, which is a turning point, where it switches which one is faster.
It might also depend on the amount of transparent bounces…
