Update column by calling function is very slow - how to improve? SQL Server - sql

Question

I need to update some column by calling a function. It takes a too long time.
How can I improve it? What could be the reason it takes so long?
I realized that scalar-function takes more time than a table-function, but I have no idea how to convert this to a table function, is there perhaps another way to do this?
The code that updates:
UPDATE EDBNationalInsuranceMortgageLoad_tmp
SET IsDead = dbo.GetIsDead(IdentityNumber1, #FileId) -- the function
The code of the function:
ALTER FUNCTION [dbo].[GetIsDead]
(#IdentityNumber1 NVARCHAR(9),
#FileId INT)
RETURNS int
AS
BEGIN
DECLARE #mateIdentityNumber NVARCHAR(9);
DECLARE #fullRow1_tmp TABLE
(
IdentityNumber NVARCHAR(9),
MateIdentityNumber NVARCHAR(9),
DeathDate NVARCHAR(8)
);
DECLARE #fullRow2_tmp TABLE
IdentityNumber NVARCHAR(9),
MateIdentityNumber NVARCHAR(9),
DeathDate NVARCHAR(8)
);
DECLARE #countRows2 INT
DECLARE #isDead1 INT = 0
DECLARE #isDead2 INT = 0
SET #mateIdentityNumber = (SELECT TOP 1 MateIdentityNumber
FROM MortgageReturnParticipationBase
WHERE IdentityNumber = #IdentityNumber1
AND MortgageReturnParticipationStatusId = 1)
INSERT INTO #fullRow1_tmp
SELECT TOP 1 IdentityNumber1, IdentityNumber2, DeathDate
FROM EDBNationalInsuranceMortgageLoad_tmp
WHERE IdentityNumber1 = #IdentityNumber1
AND IsRowError = 0
AND FileId = #FileId
SET #isDead1 = (SELECT 1
FROM #fullRow1_tmp
WHERE DeathDate IS NOT NULL)
IF #mateIdentityNumber = 0
BEGIN
IF (#isDead1 = 1)
RETURN 0;
ELSE
RETURN 1;
END
ELSE
BEGIN
INSERT INTO #fullRow2_tmp
SELECT TOP 1 IdentityNumber1, IdentityNumber2, DeathDate
FROM EDBNationalInsuranceMortgageLoad_tmp
WHERE IdentityNumber1 = #mateIdentityNumber
AND IsRowError = 0
AND FileId = #FileId
SET #countRows2 = (SELECT COUNT(*) FROM #fullRow2_tmp)
IF #countRows2 = 0
RETURN 2;
SET #isDead2 = (SELECT 1 FROM #fullRow2_tmp WHERE DeathDate IS NOT NULL)
IF (#isDead1 IS NULL OR #isDead2 IS NULL OR #isDead1 = 0 OR #isDead2 = 0)
RETURN 1;
IF (#isDead1 = 1 AND #isDead2 = 1)
RETURN 0;
END
RETURN NULL;
END
I tried to convert it to a table-function, but it worked slowly:
ALTER FUNCTION dbo.GetIsDeadTableFunc (
#IdentityNumber1 nvarchar(9),
#FileId INT)
RETURNS TABLE AS RETURN
select (case when (b.MateIdentityNumber > 0 and b.MateIdentityNumber
not in (SELECT IdentityNumber1
from EDBNationalInsuranceMortgageLoad_tmp
where IsRowError=0
and FileId=#FileId))
then 2
when (b.MateIdentityNumber = 0
and DeathDate is null)
or (b.MateIdentityNumber > 0 and b.MateIdentityNumber in (SELECT IdentityNumber1
from EDBNationalInsuranceMortgageLoad_tmp
where IsRowError=0
and FileId=#FileId
and (DeathDate is null or DeathDate is not null))
and DeathDate is null)
or (b.MateIdentityNumber > 0 and b.MateIdentityNumber in (SELECT IdentityNumber1
from EDBNationalInsuranceMortgageLoad_tmp
where IsRowError=0
and FileId=#FileId
and DeathDate is null) -- אמור להיות עוד פונה והוא נמצא בקובץ חי
and DeathDate is not null) -- פונה 1 נפטר
then 1
else 0 end) as IsDead
from EDBNationalInsuranceMortgageLoad_tmp e
join MortgageReturnParticipationBase b on b.IdentityNumber = e.IdentityNumber1
WHERE e.IdentityNumber1 = #IdentityNumber1
and IsRowError=0
and FileId=#FileId
and b.MortgageReturnParticipationStatusId=1
GO