I didn't compare head to head, but I would think something like this might perform better with longer strings:

-- NOTE: this uses the len function, so this function ignores whitespace at end also

CREATE FUNCTION dbo.LastIndexOf(

@char as char

, @string as varchar(max)

)

RETURNS integer

AS

BEGIN

SET @string = rtrim(@string)

if len(@string) = 0

return 0

DECLARE @revString as varchar(max)

DECLARE @lastIndex as integer

SET @revString = reverse(@string)

SET @lastIndex = charIndex(@char, @revString) -- find the last char in the reverse string

IF @lastIndex = 0

return 0

SET @lastIndex = len(@string) - @lastIndex + 1 -- flip it so counting from the front

RETURN @lastIndex

END