Can you define "degrade"?
Has the underlying data model changed?
One of my published workbooks has recently been miss-reporting production by a significant number. For one individual the production for one day in the published report was 11, while in an offline re-build of the workbook using the same data set was 16. I checked the filters and couldn't find anything that would indicate an issue. I know for a fact that certain fields have been added or modified on the front end, but I am not able to map the exact correlation to the fields in the tables at this point.