Dual cores will (almost) always help. This is because other processes running at the same time as your auto-route process. Windows is smart enough to assign different processes to different cores. So in the case that the auto-router can only use 1 core, ideally Windows could give the auto-router full use of a core and then give other things that need to be done (like normal overhead OS overhead) to other cores.
Writing a program to be able to use multiple cores is rather simple to do; however, it is usually split up by things like UI vs backend. It is much more difficult to split up computation heavy processes like auto-routing. The way they could do it is by having different routing methods be in different processes and then comparing when each one finishes. If I were to guess, I would be altium hasn't done this much splitting up of their program, but I don't know that for sure.
So in summary, will it help to have more cores? Yes. Will it have a huge increase in performance? Maybe. Will more cores be better then less but faster? Even harder to say. My overall recommendation would be to ask Altium. Most companies do extensive performance testing and will know their software better then any of us.