Thom,
They both resolve to extremely fast string manipulation routines in C++. The only speed difference you should be able to detect is that left() only takes two arguments vs the three for substr(). But I seriously doubt you'll see much performance differences.
>I keep seeing
>
>SUBSTR([string], 1, [width])
>
>
>instead of
>
>
>LEFT([string], [width])
>
>
>I've always assumed that the LEFT() command (with underlying C code) would be faster than interpreting (loosely speaking) the SUBSTR() command.