> I just did the test on a new build of 1.5.1_ALPHA.. and
> unfortunately performance is not measurably better.

Please verify that you have

sys/arch/i386/i386/in_cksum.s version 1.9.22.1.

The commit of that file was done between 23:00 and 00:00 UTC
yesterday, and would take some time to propagate to the anoncvs
server.

You could also double-check that this text is part of your
CHANGES-1.5.1 file:

sys/arch/i386/conf/files.i386                   1.183
sys/arch/i386/i386/genassym.cf                  1.25
sys/arch/i386/i386/in_cksum.s                   1.10-1.12

  Use optimized i386 assembly version for in4_cksum as well as the
  older in_cksum.  Gets back the 10-15% throughput loss compared
  to the NetBSD 1.4 branch.

Regards,

- H=E5vard