Processing cost of last twelve month / trailing twelve month calculations

In our rules we have multiple calculations to calculate the total or average of the last 12 months.   However as these calculations always have to refer to a different subcube they are quite costly.  At the moment about 30% of a consolidation is spent on calculating these.  We have a function set up for these calculations which is basically   YTD current month + YTD prior year end - YTD prior year / current month.   Has anyone explored a different way to do these calculations that is quicker.

Thanks,

Nils