How are you testing?
On my PII OBSD firewall I get a much lower download speed then on my Athlon 3700+ OBSD system which is behind the PII.
A couple of month my ISP increased my ADSL line from about 3000Kbit/s to 7296.
On my amd64 workstation the download speed only improved after changing
net.inet.tcp.recvspace=32768 into
net.inet.tcp.recvspace=65535
Please note that increasing the receive space only works for the receiving end, not for a router.
What is the output of ifconfig?
Code:
$ ifconfig re0
re0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
lladdr 00:19:db:47:b0:4c
groups: egress
media: Ethernet autoselect (100baseTX full-duplex,rxpause,txpause)
status: active
inet 192.168.222.20 netmask 0xffffff00 broadcast 192.168.222.255
inet6 fe80::219:dbff:fe47:b04c%re0 prefixlen 64 scopeid 0x2
Make sure it runs full-duplex and 100baseTX.