business rule performance

Summary:

Business rule calc time - any way to restructure for performance

Content (please ensure you mask any confidential information):

So, I have a business rule that takes hours to run, the issue is entirely down to this piece of logic - I know this is pulling back multiple periods' values to sum and recalc on closing rates - but is there a more efficient way of achieving this? Currently on what is essentially processing 237 blocks of data this runs in over 2 hours in a test environment - and this isn't a block creation issue - I handle that before this piece of code - which runs in isolation in that amount of time.