NAT under performing direct connection by 10x

[Malcolm Matalka]

I am setting up my Raspberry Pi Model B+ as a router and am experience
a throughput bottleneck that I cannot track down.

I'd like to preface my question with two things:

- I'm new to this so it is quite possible this is something obvious and
  I simply lack the knowledge to see it.  If so, sorry for wasting
  anyones time and thank you very much to whoever points it out to me.

- It is quite possible that my Raspberry Pi does not have the hardware
  to support the desired throughput, but I'm having trouble finding
  evidence to back up that hypothesis.

The setup I have is:

Laptop -> (wlan0 - RPi - ue0) -> Router

I am using a USB dongle with a 'run0' interface running as a hostap.
The RPi is then connected to a router which is connected to the
internet.

I have run the following command in different experiments:

wget -O /dev/null
ftp://ftp.freebsd.org/pub/FreeBSD/releases/VM-IMAGES/10.1-RELEASE/amd64/Latest/FreeBSD-10.1-RELEASE-amd64.raw.xz

On the RPi: download speed of around 500 KB/s

On the laptop: download speed of around 80 KB/s

I have also scp'd a file from the laptop to another computer on the other
side of the router:

Laptop -> RPi -> Router -> Computer

And I have a throughput of around 1 MB/s.  This exercises both network
interfaces similar to downloading from the internet.

The only difference I am aware of is that NAT is happening in the
Internet case.

I have:

- Checked CPU, in the NAT case the load average does not break 0.2.  The
  CPU does not seem stressed.

- Interrupt %, in top, doesn't go above a peak of 4% in either case.

- netstat -i and netstat -s do not show any errors happening (that I can
  see).

- While I have no idea if it is significant, I have noticed that in the
  case of a local download (on the RPi), in tcpdump the "win" goes down
  to around 1000.  In the NAT case it stays at around 4096.

Currently I am at a loss to explain the bottleneck.  Is there a
suggested next place to explore to track this down?  As I said, it could
be that the RPi is not capable of these throughputs, I'm using this as
an excuse to learn how to investigate a performance bottleneck in
FreeBSD though so I'm more interested in the meta-discussion of how to
determine where the problem is.

Thank you,
/Malcolm