UDF 10x Slower Than Select Query It Contains

I have a table valued UDF with only one SELECT query. It selects from a View, and has a couple of joins and calls another UDF in the WHERE clause. When I run the Select query itself it's duration is around 120, but when I have it wrapped in a UDF, it's around 1200!

ALTER FUNCTION [dbo].[fnGetiii]
(
bla bla bla
)
RETURNS @ItemTable TABLE
(
a varchar(2),
b bigint,
c smallmoney,
d varchar(50),
e varchar(50),
f int,
g varchar(50),
h datetime,
i int,
j int,
k int
)
AS
BEGIN
INSERT INTO @ItemTable
(a,b,c,d,e,f,g,h,i,j,k)
SELECT
vat.a AS a,
vat.b AS b,
vat.c AS c,
vat.d AS d,
vat.e AS e,
vat.f AS f,
vat.g AS g,
vat.h AS h,
vat.i AS i,
vat.j AS j,
vat.k AS k
FROM dbo.vw_jkl vat
LEFT JOIN dbo.uio uio
LEFT JOIN dbo.iop iop
ON (iop.kko = uio.kko)
ON (uio.jj = vat.jj)
WHERE this = that AND
this = that AND
this IN (SELECT iii FROM dbo.fniiireturn(pppp))

RETURN
END

How can I speed it up?