As to speed, the basic gun mount weights about the same as Phalanx (6 tonnes), Half a dozen missiles aren't going to add enough to slow it down that much.
From what I’ve read, the missiles are used to extend the range of the CIWS system - all CIWS systems have a relatively short range inherent to their smaller caliber rapid fire guns. The missiles are to be used to engage oncoming threats beyond the range of the gun and the gun fires when the threat enters its range if the missiles haven’t knocked it down.