Is there a performance benefit or cost related to IPv6?

IPv6 is designed to reduce the processing done by intermediate routing performed by the intermediate routers while forwarding the traffic. The aim is just look at the destination in the routing table and forward. In between routers dont have to fragment the packets and NO checksum checking(left for upper layers to verify). But as WindowsUser mentioned above, IPv6 routing lookup might be slow because of long IPv6 addresses.

This is a very complex question dependent on many factors. You must consider the IPv6 connection type in question, its speed, the underlying IPv4 connection, the size of the network, and the routing policies of the network.

• IPv6 improves performance when compared to NAT.
• IPv6 decreases performance if it is over a rather long-distance tunnel, or if it is a broken implementation
• IPv6 decreases performance if its connection is not as fast as IPv4 (obviously)
• IPv6 decreases performance on large network routers because of the increased overhead from processing the vastly larger routing table
• The efficiency of IPv6 and IPv4 on a network is determined by the routing protocol used by the routers within the network.