User Defined Data Type Used In Stored Procedure Parameters

I have several stored procedures with parameters that are defined with
user defined data types. The time it takes to run the procedures can
take 10 - 50 seconds depending on the procedure.
If I change the parameter data types to the actual data type such as
varchar(10), etc., the stored procedure takes less that a second to
return records. The user defined types are mostly varchar, but some
others such as int. They are all input type parameters.
Any ideas on why the stored procedure would run much faster if not
using user defined types?

Using SQL Server 2000.

Thanks,
DW