That is odd. If anything, it would seem like it would slow it down (mysql optimizer has to think about what to do with extra indices?). My suggestion is create the table like so:
CREATE TABLE `posts` (
`pid` int unsigned NOT NULL auto_increment,
PRIMARY KEY (`pid`),
I removed the not null "constraint" from the text portion because you will most likely be using asp/php/perl to check to see if it's empty before you allow the post anyway... (and you're checking the return value from the insert anyway).