[imp] IMP/Horde slowness

[서진수]

If all of performance tips in "horde/docs/PERFORMANCE"
fail to improve Horde/IMP performance,
try to change "horde/imp/config/conf.php":

$conf['mailbox']['show_attachments'] = false;
$conf['mailbox']['show_preview'] = false;
$conf['mailbox']['show_xpriority'] = false;

In my case, speed of Horde/IMP is near to that of SquirrelMail.