Each time you use an aggregate function call, the mapping searches and groups the data. For example, in the following expression,

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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 shown below,

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adds COLUMN_A to COLUMN_B, then finds the sum of both:

SUM(COLUMN_A + COLUMN_B)