The mission control plugin stores activity session activity in it's jos_rokuserstats. Just look in that table for what's stored. Basically userid, ip, sessionid, page and timestamp.
Speed should not be an issue. The table is indexed, and there's much more going on with the Joomla side of things. You can always test in your instance by disabling the MC system plugin temporarily and seeing if it makes any difference on your site performance. It really should not have any discernable effect.