[PERFORM] Fastest char datatype

[Robert James]

I'm storing a lot of words in a database.  What's the fastest format for
finding them? I'm going to be doing a lot of WHERE w LIKE 'marsh%' and WHERE
w IN ('m', 'ma').  All characters are lowercase a-z, no punctuation, no
other alphabets.  By default I'm using varchar in utf-8 encoding, but was
wondering if I could specificy something else (perhaps 7bit ascii, perhaps
lowercase only) that would speed things up even further.