That is actually a very good speed.  During the transition to the policy or vlan that is assigned after the registration occurs, you must remember that there is a re-auth that takes place, and hence another IP Resolution process that also occurs, prior to NAC redirecting the page to whatever URL.  That is a good time;  some take upwards of 30 seconds.  Everyone’s networks is different and if they are changing vlans that can exacerbate the delay.