Can you post code?

There are a number of constructs within functions that look good and perform terribly. As an example, if you have a scalar udf (user-defined function) that reads 10 rows and you use that function in the select clause of a query that returns 10 000 rows, then the function runs 10000 times (in most cases) and those 10 rows that it reads are each read 10000 times.