Here is an example of how to properly benchmark an operation on distributed arrays:
You are comparing the implicit parallelism (multithreading) of core MATLAB to the explicit parallelism (MPI communication between MATLAB workers that are themselves running singlethreaded) of the Parallel Computing Toolbox distributed arrays. If the input matrices A and B fit in memory on a single machine with enough memory left over to perform the operation and store the output, it's often pretty hard to beat the performance of the multithreaded operation. Distributed arrays provide a clear benefit when the problem exceeds the memory available on a single machine.
If you set up a benchmark for mtimes (*) following the example that I've linked, you can investigate the performance you can expect from your own machine. I think that the codistributor you have chosen is not unreasonable for this operation.
