Query / Criteria ?

See the attached Function.
tblGuides is about 5M rows. tblPOSupplierItemList 377.
When I run this function it takes < 1 second and returns 732 rows.
But when I add this criteria to the Where it takes more than 10 minutes and returns 75 rows. It does not matter where I add this criteria, inside the cte, outside or even in ssms (select * from dbo.POItemsBySupplierList(.5) <Where adding it here>). Is there a way to tell SQL to only apply this criteria to the result of 732?

Criteria:
((g.[New Net Pr] - ((sl.NetPrice ) + @ShippingCost)) / ((sl.NetPrice ) + @ShippingCost)) > 0

ALTER FUNCTION [dbo].[POItemsBySupplierList] 
(	

	@ShippingCost smallmoney = 0

)
RETURNS TABLE 
AS
RETURN 

With CTE As(

Select	g.ISBN, 
		g.Title,
		g.Publisher,
		g.[new net pr]								As	GuidePrice,
		dbo.QtyLeft_Guide(g.GuideNumber, g.ISBN)	As	QtyLeft,
		sl.NetPrice									As	Cost,
		sl.Discount,
		sl.LeadTimeDays,
		@ShippingCost								As ShippingCost
		

	From	tblGuides g
	Join	tblGuideHeader gh			On g.GuideNumber = gh.GuideNumber
	Join	tblPOSupplierItemList sl	On g.ISBN = sl.ISBN

	where	(DateDiff(day, GetDate(), gh.FinalShipDate) >= sl.LeadTimeDays)
		And
			(dbo.QtyLeft_Guide(g.GuideNumber, g.ISBN) > 0)
		--And
		--	((g.[New Net Pr] - ((sl.NetPrice ) + @ShippingCost)) / ((sl.NetPrice ) + @ShippingCost)) > 0
)
select  * from cte
--where ((GuidePrice - ((Cost ) + @ShippingCost)) / ((Cost ) + @ShippingCost)) > 0

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