Hi,

I've recently experimented with persistent connections on my main site (not ipb, all custom code) and it really delivered noticeable improvements. This feature was (re)introduced in PHP 5.3, perhaps we can have an option in IPB to use it on systems that are capable of doing so. More info

http://php.net/manual/en/mysqli.persistconns.php
http://blog.ulf-wendel.de/?p=211

All you should have to do is edit your sql_host variable in your conf_global.php file to add "p://" before it. i.e. instead of "localhost" make it "p://localhost" (at least according to the documentation you linked to - I haven't experimented yet).