This is about twice as fast for me:

CREATE ASSEMBLY Test

FROM 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

WITH PERMISSION_SET = SAFE;

GO

CREATE FUNCTION dbo.SumDigits

(@Input integer)

RETURNS integer

AS EXTERNAL NAME Test.UserDefinedFunctions.SumDigits;

GO

-- Test & ensure function is fully compiled

SELECT dbo.SumDigits (2147483647);

SET STATISTICS TIME ON;

DECLARE @T INT;

SELECT @T = dbo.SumDigits(TestNumber)

FROM #Test;

SET STATISTICS TIME OFF;

Source:

[font="Courier New"]    public static int SumDigits(int Input)

    {

        int sum = 0;

        for (int n = System.Math.Abs(Input); n > 0; sum += n % 10, n /= 10) ;

        return sum;

    }

[/font]