MATLAB uses built-in parallelism on a single machine for many large matrix operations. See this page: https://uk.mathworks.com/discovery/matlab-multicore.html . Parallel Computing Toolbox workers run in a single-threaded mode (by default, but this can be configured by changing the NumThreads property in your cluster profile), so each computation on a worker will be slower, but you can do more individual computations at a single time.
The general rule is that: if MATLAB's built-in parallelism applies to your problem, it will be faster than using PCT to solve the same problem since it does not have to copy data to/from the worker processes.
