If we put functionality such as calculations in FM instead of a Report, does it improve performance? I have got conflicting views on the matter. I personally feel it shouldn't
The real difference is where calculations are performed (database / Cognos server)
When you define for instance running totals, then the Cognos server chimes in. If Cognos can compile a calculation that can be handled by the database then it would be part of the regular SQL executed on database level. It should then not matter WHERE the actual statement comes from, apart from the possibility that execution paths may not yet exist