Slow NAT on Windows Server 2003

Hi,

Network Address Translation (configured in "Routing and Remote Access")
under Windows Server 2003 seems to be very slow and I'm not sure if this
is normal or not?

If I run a speedtest from the server running NAT, I can download at
15Mbps. However if I download from any computer behind this machine on
it's internal interface, I only get a 4 or 5Mbps download speed. The
whole network is running at 100Mbps or above and I have verified that I
can download a file from the NAT server at close to 100Mbps, so it's not
the LAN network that's causing the slowness.

Any ideas what the problem might be? Is NAT under Windows really really
slow? The CPU is not highly loaded (5-10%) while the speed test is running.

How can I diagnose/fix this please?

Nick....