Function Doubt

Dear All,
i've a function like this
CREATE FUNCTION f1 (@inId CHAR(2))
RETURNS VARCHAR(20)
AS
BEGIN
DECLARE @strName AS VARCHAR(50)
SET @strName =
CASE @inId
WHEN '1' THEN 'Active'
WHEN '2' THEN 'InActive'
WHEN '3' THEN 'Closed'
end
return @strName
end

while we are using this in application, it is taking almost 20Min of time.

sometimes the input is going to be null. is that be a reason for this?

then can i put there
else null

thank you in advance for the time