When writing expressions, factor out as many aggregate function calls as possible. Each time you use an aggregate function call, the Integration Service must search and group the data. For example, in the following expression, the Integration Service reads COLUMN_A, finds the sum, then reads COLUMN_B, finds the sum, and finally finds the sum of the two sums:
SUM(COLUMN_A) + SUM(COLUMN_B)
If you factor out the aggregate function call, as below, the Integration Service adds COLUMN_A to COLUMN_B, then finds the sum of both.
SUM(COLUMN_A + COLUMN_B)