Serious Perfomance Problem Using UDFs As Function Arguments. (I Suspect A Bug, Show Me I'm Wrong!)

(From an exchange originally posted on SQLServer.com, which wasn't resolved...)

To return views tailored to the user, I have a simple users table that holds user IDs, view names, parameter names, and parameter values that are fetched based on SUSER_SNAME(). The UDF is called MyParam, and takes as string arguments, the name of the view in use, and a parameter name. (The view the user sees is really a call to a corresponding table returning UDF, which accepts some parameters corresponding to the user.)

But the performance is very dependent on the nature of the function call. Here are two samples and the numbers reported by (my first use of) the performance monitor:
Call to table returning UDF, using local variables:

declare @orgauth varchar(50)
set @orgauth = dbo.MyParam('DeptAwards', 'OrgAuth')
declare @since datetime
set @since = DATEADD(DAY,-1 * dbo.MyParam('DeptAwards', 'DaysAgo'),CURRENT_TIMESTAMP)
select * from deptAwardsfn(@orgauth,@since)

[187 CPU, 16103 Reads, 187 Duration]


Call to same table returning UDF, using scalar UDFs in parameters:

SELECT *
from deptAwardsFn (
dbo.MyParam('DeptAwards', 'OrgAuth')
,DATEADD(DAY,-1 * dbo.MyParam('DeptAwards', 'DaysAgo'),CURRENT_TIMESTAMP)
)
[20625 CPU, 1709010 Reads, 20632 Duration]
(My BOL documentation claims the CPU is in milliseconds and the Duration is in microseconds -- which I question.) Regardless of the unit of measure, it takes a whole bunch longer in the second case.

My only guess is that T-SQL is deciding that the parameter values (returned by dbo.MyParam) are nondeterministic, and continually reevaluates them somehow or other. (What ever happened to call by value?)

Can anyone shed some light on this strange (to me) behavior?


----- (and later, from me)---
(I have since discovered that the reference to CURRENT_TIMESTAMP in the function argument is the cause, but I suspect that is an error -- it should only capture the value of CURRENT_TIMESTAMP once, when making the function call IMHO.)