Having a unitary matrix $X$ whose size is $n \times n$ and a vector $z$ whose length is $n$, and let's have:
$$y = X^H {\rm diag}(z)X$$
where $X^H$ is the conjugate transpose of $X$.
My question, is the complexity of this multiplication is still almost $n^3$ or since the matrix $X$ is unitary, multiplication complexity should be less ?