@desk_jedi 

If you were to wire your Xbox to the TP-Link router, what speeds do you get?

Sorry to the late reply. Guess my alerts don't work. Here are the results of my tests:

Wired:

Wireless:

As you can see, the upload speeds are nearly the same but there is a wicked disparity between download speeds. If you suspect the Wifi adapter is failing, it isn't. If I re-enable the wireless on my CPE and connect through it the download speeds look like those when wired. It's like the Archer C9 is throttling it back.

@desk_jedi 

There is one other setting you could try, that is NAT Boost which is located at Advanced > System Tools > System Parameters, scroll down.

If that does not yield any results, I would suggest contacting our support as they will be able to research your case further.

Thanks, Tony but it didn't make a difference. I'll open a ticket with support. Thanks,

I'm assuming you want me to change the MTU value on my router because there isn't a way to change it on the Xbox 360 that I know of. It is not in AP mode. Devices on the network only connect to the AC1900. It in turn connects to the Actiontec which is connected to the Internet. The TP-Link acts as a DHCP server, print server and secondary firewall for the LAN which is on a different subnet than WAN facing side of the AC1900.

Chaning the MTU size from the default 1500 to 1472 as you suggested had no effect on the download speed. I compared it to the MTU setting on the Actiontec and confirmed that it is also set to 1500 so MTU isn't the cause here. NO, this router is bottlenecking traffic from the Xbox 360 and I want it fixed. I'd like to know why as well but I'll settle for it being fixed. I've opened a ticket with support but apprently everyone is on holiday until the fourth. Guess I'll just have to wait...

So for anyone who might still be watching, or happens upon this thread researching a similar issue, the result of all this turned out to be the router failing. After basically being told by support, "sorry, we can't help you. Use a cable instead!" the WAN port on the C9 went kaput and the router is being replaced. We'll see what happens when the replacement arrives but if results are the same I'm going out a buying another router. Overall, I've found the Archer C9 to be a bit underwhelming in terms of overall performance and flexibility. It's Wi-Fi bands are supposed to be stronger than my CPE, but when placed side by side the CPE's range covers almost my entire property( ~.333 acre) and the C9 barely covers the whole house (~1400 sqft more or less in the middle of the property). That's for both the 2.4GHz and 5GHz bands. Using the router as cloud storage was frustratingly slow but I only had an older USB2.0 thumb drive to test with so it may not have been the routers fault. I still think it may bog down trying to manage large file transfers regardless of storage media quality though, just speculation. The printer server feature is flashy but isn't really a printer server. You still have to install an app on your PC that has to be running to print. Why? Let the router's firmware handle that. All of my wired connections performed well both online and over the LAN but that wasn't why I chose this router, wireless was. I hope the replacement changes my mind otherwise the C9 is going on eBay and I'm finding another router vendor.

@Carl 

Just so it's clear, I've been operating in this configuration or some variation thereof for over 15 years with no problems until recently and only with the 360. Double NAT has never been a problem before now (nor should it ever be, businesses all over the world use much more complicated configurations today without issue), and that was using routers by Linksys, Dlink and Asus over the years. Furthermore double NAT is not the purpouse of this thread. I have at the moment of my writing this post no less than 7 devices connected via wi-fi and 3 of those are on the 2.4GHz channel. One if those is the Xbox which is now crawling along at it's usuall 9/3 Mbps rate. The other two wireless devices are the printer and an HP Pavilion laptop which speedtests at 40/40 Mbps. 

Now, as it so happens, I've ordered an external N wi-fi adapter for the 360 so I can either confirm or rule out it's wireless adapter as the point of failure. I've also had my ISP replace the Actiontec with a newer FiOS-G1100 unit.

Current configuration: Internet >> (DHCP) FiOS-G1100 Gateway (STATIC) >> (STATIC) Archer C9 (DHCP) >> (DHCP) LAN

I know the the "print server" feature isn't necessary for network enabled printers but the one we have is an older wi-fi model (no Ethernet) and our Chromebook seems to find it on our network more reliably when it is hosted by a machine and shared. I know, I know, the only real "cloud-based" PC we own can't reliably use the cloud to print. The irony isn't lost on me.

You cannot adjust the MTU settings on the Xbox 360. Not without modding the console anyway, so no. Now you know, make a note.