Cisco WAN :: 2651XM - Slower Network Routing Only With OS X

We have a Cisco 2651XM at the edge of our network, which routes our public IP block. This sits on a 100 Mbit/s ethernet pipe (full duplex) up in the datacenter.  We are also running 100mb full duplex on our our side of the network.  We have several public servers behind the router. 
 
We have recently set-up a new Apple OS X Lion Server to serve a few websites.  However, when downloading some files from the server the from a remote location, I noticed I can only get a maximum of 15 Mbit/s out of the connection. 
 
When downloading a large file over HTTP (Apache) from this OS X Lion  server from another server on the same internal network (behind the  router), we get a full 100 Mbit/s transfer speed.
 
However,  when we download the same file from anywhere on the internet (external  side of the router), we can only manage to get 15 Mbit/s out of the  transfer.
 
However,  we also have other Linux & Windows servers that we can achieve a  full 50 Mbit/s (our office connection speed) externally under the same  network conditions.
 
So it  also appears it's only for a single connection - not a limitation for  the whole server.  If I open two HTTP connections, I can get say 15  Mbit/s out of each transfer - totalling 30 Mbit/s... if that makes  sense.
 
Update: I also notice slower ping latency when pinging from outside the network. Most of the servers reply in 15ms while the new OS X server usually takes over 40ms on average.
 
Router show ver:
Cisco Internetwork Operating System Software
IOS (tm) C2600 Software (C2600-IS-M), Version 12.2(11)YT2, EARLY DEPLOYMENT RELEASE SOFTWARE (fc1)

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