Is your WLAN using a VC-Assigned VLAN? In that case, all of the guest traffic is tunneled to the VC, so there has to be a VC election to decide who will handle and nat that traffic. The minimum failover time should be 10 seconds, really. If you are achieving 5 seconds, that is good.
If you were using 802.1x and you had a network-assigned VLAN, the AP a user is assigned to would just keep passing traffic, even during the VC election, because the data traffic would be truly distributed.