calculating YTD from MTD data

Hi,

As the data is loaded in MTD values I want to calculate the YTD value for the Balance Sheet accounts in EPM Planning. The dynamic time series only considers the current month, but I also need the previous years to be taken into account. That's why I created to custom rule which first copies the MTD member to the YTD member and then calculates the YTD values. The issue now is that this rules takes at least 4 hours before it is completed. Does someone has a suggestion on how to adapt this rule in order to make it faster?