I'm not sure that the two concepts are resulted at all. Obviously mongo has map reduce baked in - but that's not that relevant. Map/reduce is a reasonable paradigm for crunching information. I have a heavily CPU bound system that I parallelise by running on different machines and aggregating that results. I probably wouldn't call it map reduce - but really it's the same thing.

How do you parallelise your long running tasks otherwise?

I can't say without more information on the problem to solve. As I said above, there are cases where MapReduce is a good tool.

And even if you improve the DS/Algorithm first, usually that is usable by the MapReduce port and you save a lot of time/costs.