i386 home firewall/router/nat bottleneck diagnostics?

[yancm%sdf.lonestar.org@localhost]

Hi All,

I just upgraded my internet service from
dsl (3M/512K) to Comcast business class cable (20M/5M).

We have demonstrated that in front of my firewall
box (description below) the service delivers
20M down / 5 M up as advertised. The "modem" is a SMC combo
that I cannot get into bridged mode, but I think I can
live with that for the cost... I do not think that is the issue.

My server/firewall/nat/router box is an i386 NetBSD 5_Stable,
300 MHz PIII w/512M Ram.

My internal network switch is a 10/100/1000 capable unit.

I run ipnat/ipf in a similar configuration to what
I've been running for 10 years. I had to make the
obvious adjustments to work with the brain dead modem/router
they gave me, but my block log is now clean...I can connect to
everything as I could on the DSL service...

Network cards:
- to the cable modem I have a realtek 8169SC
  which uses the re driver. Mode is 1000baseTX.
  ID as re0... Registers fine with kernel dmesg
  looks good in ifconfig as well.

- to the internal network I have a realtek 8139
  which uses the rtk driver. Mode is 100BaseTX.
  ID as rtk0... Registers fine with kernel dmesg
  looks good in ifconfig as well.

From inside my firewall, I can only achieve about 5M down / 3M up.

Any pointers on diagnostics, how-to's (I've read most
the ones I could find) would be deeply appreciated.

Is my i386 hardware too slow?

Please help me obi wan!