Are these multi-statement table valued functions? If so, understand that this type of function has not statistics. This means the optimizer treats them like a table that returns one row. That's OK when the number of rows returned is relatively low (depending on your data, say less than 100 rows). But when you get to larger sets of data or you start doing things like joins or unions between multiple UDF's like this, you will see a serious performance penalty.

If not, can you post the execution plans for the UDF's?

Either way, are you sure you need to use a UDF to return this data? Even simple table valued functions can underperform regular TSQL queries.

Here's a post outline some details around this:

http://sqlinthewild.co.za/index.php/2008/08/12/views-or-functions/[/url]

And another, just not as detailed:

http://scarydba.wordpress.com/2008/08/13/view-vs-table-valued-function-vs-multi-statement-table-valued-function/[/url]