Speed up Running Total MDX calculated measure?

Question

Hi all, I'm using the follow mdx to keep a running total of the Period Balance measure in my cube:

SUM({[Due Date].[Date].CurrentMember.Level.Item(0):[Due Date].[Date].CurrentMember}, [Measures].[Period Balance])

It works great, however it's really slow as the amount of data displayed increases. I can't use a MTD or YTD because the users may be analyzing data that overlaps years. Any way I can speed this up?

Thanks in advance.