Hello Wayne,
NSF use can lead to worse results in some cases.
With NSF you are asking to neighbor device to ignore the fact you have stopped to send OSPF hello messages and to hide the switchover.
It is probably better in your case to avoid to use NSF and use if possible small OSPF timers (sub-second hello messages)
I see NSF a good fit for a C6500 or C4500 with two supervisors installed on it.
To complete information you should provide the IOS image name running on the stack, the configuration of the stack and that of the neighbor devices.
Also the STP mode you are using on the client vlans, if in use can have an effect.
I guess you are measuring the convergence time from client vlan to remote IP subnet behind the C6500 devices.
Hope to help
Giuseppe