SUM and LOOKUP used in the same expression generally cause large formula calculations and may cause intermediate relationship calculations especially if Time is a dimension or when the source and target structures are very different. Splitting them into different modules and line items considerably reduces the size of the intermediate calculations
Exception:
2.02-08a The LOOKUP is a constant: Where the LOOKUP line item is a constant, the formula will operate as a SELECT, so the performance implications are negated
Best Practice article:
Formulas and their effect on model performance
Questions on this topic? See 2.02-08 Never use SUM and LOOKUP together in our Planual forum.