Nested dissection for solving x=A\b

I am using nested dissection ordering with multi-level Schur complement procedure for solving x=A\b. The linear system arises from FEM discretization of 2D poisson problem. Nested dissection should be faster than backslash for large problem sizes. (Please see https://www.mathworks.com/matlabcentral/newsreader/view_thread/241438)

But in my experience I found that for small problems they were both taking almost same time and nested dissection tends to slow down for large problem sizes which should not happen. So what should I do in order to correctly implement the nested dissection method in matlab to get the speedup for large problems than backslash.