Performance issue with UDF suring bulk loads (SSIS)

Hi All,
We have bulk load jobs configured using SSIS packages. All the load jobs were working perfectly fine until 2 days back. Suddenly, our load jobs are taking way longer than the normal time. The issue is identified to be the use of UDF as shown below,
Note: These jobs were working fine since 3 years without any issues. We load around 16 GB of data each day.
No changes to the server and\or SQL Server itself.
Any help would be greatly appreciated.

Thanks,
-Mohsin
ALTER FUNCTION [dbo].[convertdate_null] (@input varchar(50))
RETURNS VARCHAR(12)
AS
BEGIN
DECLARE @output DATETIME;
SELECT @output = CASE WHEN (ISDATE ( ( CASE WHEN Left(replace(ltrim(rtrim(@input)),'+',''), 1) =1
THEN '20'
ELSE '19'
END + Right(ltrim(rtrim(@input)), 6))
) = 1)
THEN CONVERT(VARCHAR(12),CONVERT (DATETIME, CASE WHEN Left(replace(ltrim(rtrim(@input)),'+',''),1) = 1
THEN '20'
ELSE '19'
END + Right(ltrim(rtrim(@input)), 6) ) )

ELSE
null
END
RETURN @output
END