I need to calculate a matrix multiplication that , where B is a full matrix with and D is a digonal matrix with .The computational complexity is
Acturally, if the matrix D is a full matrix, the computational complexity will be .
I recorded the running time for both cases in matlab, and find that the running time and time complexity are not consistent, how can I speed it up? I want to use less time to calculate the first case compared with the second case.