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.