Oracle LIKE vs. SUBSTR evaluation

You learn something new every day. Today after a fair amount of testing, I discovered that...

SELECT
    *
FROM
    MYTABLE
WHERE
    ID LIKE 'STRING%'

...is about about twice as fast as

SELECT
    *
FROM
    MYTABLE
WHERE
    SUBSTR(ID,1,STRINGLENTH) = 'STRING'

YMMV.