How to speed up force calculate step?

The system already parallelizes everything it can. There isn't a magic "go faster" button.

Force Calculate will run on all previous periods, ignoring calculation status of every dataunit. So, in December, you're recalculating the whole year.

That, plus (likely) inefficient rules, can result in bad performance like you're seeing. 

You'll need to look at the performance of your rules, scouring the log for timings (you can use the With Logging option for more details, although it will slow down things further) and checking the various Application Reports for dense dataunits, bad rules using #All, excessive reliance on Business Rules rather than Member Formula, etc etc.