Xserve G4 - NAT has slow performance. Why?

Question

I 3 Linksys Routers as access points. One right now is the gateway and DHCP server, the 2 others are access points around the house. I have an Xserve G4 (RackMac 1,1) tray loading, dual 1GHz G4 PPC CPUs and 1.5 GB of RAM. When I use the Xserve as the gateway, DNS, and DHCP server using NAT Setup Assistant, performance slows from 11MB/s, to 3MB/s download speed, and almost no upload speed for the clients on the LAN. I can do a speed test on the Xserve itself, and speed is at 8MB/s download, and 1MB/s upload, so normal, but slower. I wonder if it's DNS, but when I use Network utility on a client to resolve the IP, it takes no more than a second, if even that. It might just be the hardware. On the client, web pages take second to load, then start to load, stop, start again and once the page is there, it takes a while for the rest of the objects to load. When I am on the regular Linksys Router, this is not an issue. I use Google's DNS, and my ISP's as the forwarders. But I ask, what might really be the issue? Any ideas are appreciated.

Answer 1

Are you running NAT on the Linksys as well as on the Mac? That's going to be problem number 1 - double-NAT is just so icky :: shudder ::
Second to that I'd hazard that there's a MTU mismatch that's causing a stall and buffer issue with traffic.
The MTU is the maximum Transfer Unit - the size of the largest packet that can traverse the network. It's typically 1500 bytes on an ethernet network, but can drop to as low as 1200 bytes on different internet connections. If you're using a VPN, that drops the MTU, too (since the VPN has to add its own headers to each packet).
If the MTUs are mismatched (e.g. the client tries to send a 1500-byte packet down a link that only supports 1400 bytes) then something has to give - typically some combination of buffering, rewriting or packet dropping.
The Linksys may be smart enough to publish the public MTU to the internal LAN, but Mac OS X does not. Therefore the Mac is probably telling the (internal) LAN traffic to use a 1500-byte MTU while the (external) WAN link is lower.
There are various semi-technical ways of finding out the MTU for any given link, but the easiest option is often trial and error - force the internal interface to use a lower MTU and adjust it up and down until you find the optimal size. Common sizes to try are 1480, 1460 and 1400 bytes.
You can use System Preferences -> Network -> (interface) -> Advanced to set the MTU away from its default 1500.