きゃ。これか。

FreeBSD 7.0 added TCP send and receive buffer autotuning.
There are some additional settings to modify.
(The default for these is 256 KB, which is too small):

net.inet.tcp.sendbuf_max=16777216
net.inet.tcp.recvbuf_max=16777216

デフォルトは 262144。
これは確かに小さすぎる気がする。

net.inet.tcp.recvbuf_max: 262144
net.inet.tcp.sendbuf_max: 262144