SQL XML :: Scalar Valued Function Slow Querying XML Passed As Parameter

I have a procedure that calls a SVF to convert an xmldocument. The ultimate purpose is to update the xml in a column in a multi-million row table. The xml is stored as varchar(MAX), it was supposed to carry any type of text, initially at least.

My question is: why is the xml-parsing performed inside the function much slower when i pass the xmldocument as type xml than when it is passed as varchar(MAX) and the CAST to xml is within the function? Does processing the xml input parameter in SlowFunction involve expensive crossing of some context border?

The two versions of the SVF (they return the rowcount in this simplified example):

CREATE FUNCTION [dbo].[FastFunction]
(
@inDetaljerText varchar(MAX)
)
RETURNS int

[Code] ....

The two versions of the SP

CREATE PROCEDURE [dbo].[FastProcedure]
AS
BEGIN
SET NOCOUNT ON;
select
dbo.FastFunction(al.Detaljer)

[Code] ....