Ansys Structural models do not always solve in less time when you add more cores. There are diminishing reductions in elapsed time when going from 4 to 8 to 12 cores and at some point, the elapsed time will increase instead of decrease because the overhead of dividing the problem to run on all those cores and reassembling the results takes more time than is saved.

Different models will have different benefits of adding cores.

It is nearly always better to run Distributed Ansys than to turn that off.

You don't have to do anything to reassemble the result files, that is automatically handled for you.