this suggests that turning off IPv6 might increase performance.
So I tried it. And in a gigE configuration I could not find a difference.
with IPv6:
PowerBook G4 A15 SGI Tezro
[ 3] 0.0-10.0 sec 863 MBytes 724 Mbits/sec
[ 3] 0.0-10.0 sec 760 MBytes 638 Mbits/sec
[ 3] 0.0-10.0 sec 708 MBytes 594 Mbits/sec
PowerBook G4 A15 Linux x86
[ 3] 0.0-10.2 sec 484 MBytes 397 Mbits/sec
[ 3] 0.0-10.0 sec 584 MBytes 490 Mbits/sec
[ 3] 0.0-10.2 sec 568 MBytes 467 Mbits/sec
without IPv6:
PowerBook G4 A15 SGI Tezro
[ 3] 0.0-10.0 sec 788 MBytes 661 Mbits/sec
[ 3] 0.0-10.0 sec 744 MBytes 624 Mbits/sec
[ 3] 0.0-10 0 sec 872 MBytes 731 Mbits/sec
PowerBook G4 A15 Linux x86
[ 3] 0.0-10.0 sec 581 MBytes 487 Mbits/sec
[ 3] 0.0-10.2 sec 608 MBytes 500 Mbits/sec
[ 3] 0.0-10.0 sec 598 MBytes 502 Mbits/sec
Your Mileage May Vary