Sounds like a Gratuitous ARP issue (which is the default setting), do you have VMAC set on the cluster object? That should help but if you still experience a 10-12 second delay upon failover even after setting VMAC you'll need to set portfast (NOT disable STP) on the switch ports the firewalls are connected to.
If everything is working properly, upon failover you should see the following traffic behavior:
Catastrophic Failover (active completely dies/crashes): Outage of up to 2.5 seconds
Non-Catastrophic Failover (active interface failure, clusterXL_admin down, etc.): Outage of up to 300 milliseconds
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