transform a function into a simple query - sql-server

Question

I have the following function that returns parcel's delay:
ALTER FUNCTION [dbo].[Delay]
(#DeliveryTime int, #DeliveryHour int = 0, #Product_ID int, #Country varchar(2), #ZipCode varchar(10)
)
RETURNS int
AS
BEGIN
DECLARE #Delay int, #Guaranteed Int, #MaxDeliveryTime int, #MaxHour int, #test int
if #Country = 'FR'
begin
select top 1 #Guaranteed = pz.TProduct_Guaranteed,
#MaxDeliveryTime = pz.TProduct_delay,
#MaxHour = pz.TProduct_HourMax
from TProductZone PZ
where (pz.Country_ID = '0' OR pz.Country_ID = cast(#Country as varchar(2)))
And (Zone_ID = '0' or Zone_ID = left(#ZipCode,2))
And p.tproduct_id = #Product_ID
Order by pz.country_id desc, zone_id desc
end
Else
Begin
select top 1 #Guaranteed = pz.TProduct_Guaranteed,
#MaxDeliveryTime = pz.TProduct_delay,
#MaxHour = pz.TProduct_HourMax
from TProductZone PZ
where (pz.Country_ID = '0' OR pz.Country_ID = cast(#Country as varchar(2)))
And (Zone_ID = '0')
And p.tproduct_id = #Product_ID
Order by pz.country_id desc, zone_id desc
End
if #Guaranteed = 1
Begin
if #DeliveryTime >= #MaxDeliveryTime
Begin
set #Delay = #DeliveryTime-#MaxDeliveryTime
if #DeliveryHour > #MaxHour
begin
set #Delay = #Delay+1
end
End
else
Begin
set #Delay = 0
end
End
Else
Begin
set #Delay = 0
End
return #Delay
END
This function is used as following:
Update TShipping
Set Delay = Delay(S.TShipping_Deliverytime, cast(left(isnull(TD.Tracking_Hour,0),2) as int), S.TProduct_ID, S.Country_Code, S.ZipCode)
From TShipping S
inner Join TrackingEndDelay TD On TD.TShipping_ID = S.TShipping_ID
WHERE IsNull(TShipping_Delivered,0) = 1
And DateDiff(day,TShipping_Date,getdate()) < 90
I want to transform this function (and the call of the function) in a unique simple query in order to decrease the response time.
This is the query I did:
Update TShipping
Set Delay =
case
WHEN TProduct_Guaranteed=0 or (TShipping_Deliverytime - TProduct_delay < 0) then 0
WHEN (TShipping_Deliverytime - TProduct_delay >= 0) and cast(left(isnull(TD.Tracking_Hour,0),2) as int) <= isnull(TProduct_HourMax,24) then TShipping_Deliverytime - TProduct_delay
when (TShipping_Deliverytime - TProduct_delay >= 0) and cast(left(isnull(TD.Tracking_Hour,0),2) as int) > isnull(TProduct_HourMax,24) then TShipping_Deliverytime - TProduct_delay +1
end
from tshipping TS
inner Join TrackingEndDelay TD On TD.TShipping_ID = TS.TShipping_ID
left join dbo.TShipping_DateInDelivery DID on ts.tshipping_id=DID.tshipping_id
outer apply (select top 1 pz.TProduct_Guaranteed, pz.TProduct_delay, pz.TProduct_HourMax
from TShipping S
inner join TProductZone PZ on S.tproduct_id = pz.tproduct_id
where (pz.Country_ID = '0' OR pz.Country_ID = S.Country_Code)
And (Zone_ID = '0' or (Zone_ID = left(S.TShipping_ZipCode,2) and S.Country_Code='FR'))
and TS.TShipping_ID=S.TShipping_ID
Order by pz.country_id desc, zone_id desc) r1
WHERE IsNull(TShipping_Delivered,0) = 1
And DateDiff(day,TShipping_Date,getdate()) < 90
The problem is that I increase the response time instead of decreasing it. Has someone another better way to do it faster??
Thank you for your help!!