Linksys WRT110 slow speed using PPPoE and QoS

I recently moved to a new house and got 12 mb/s DSL. In a hurry to get everything up-and-running, I left the Gigaset Speedstream 4300 DSL modem in router mode and let my Linksys WRT110 router pick up a DHCP address from it. Speed tests showed I was getting at least 12 mb/s which was awesome, but this configuration results in a double NAT which inhibits things like remote SSH to my home network.

After getting settled in the new digs, I went back and put the DSL modem in to bridge mode using these instructions from Windstream. I configured the WRT110 to initiate the PPPoE connection, and all was well. Or so I thought. Upon performing another speed test, I realized that I was getting less than 6 mb/s throughput on my 12 mb link. Going back to the double NAT config, speed test results went back to 12 mb/s. Puzzling. So after a week of futile searching and tinkering, I decided to go back to basics. With the modem in bridge mode, I connected my Win 7 laptop directly to it and connected using Win 7's PPPoE client. Voila! 12 mb/s. So it's the router!

There wasn't much else I could change on the router, so I just said "screw it" and reset it. With everything at default, I immediately connected over PPPoE at 12 mb/s! Next, I started rebuilding my configuration from scratch with speed tests after each change and as soon as I hit the QoS settings, it dropped back down to under 6 mb/s. I have a UMA-capable Blackberry, so I had it's MAC address listed as a high-priority QoS device which may or may not be necessary since it never seemed to improve the awful quality of my WiFi calls. When I turned QoS off, the speed shot back up to 12 mb/s. So that's it. Just make sure QoS (specifically, the Internet Access Priority settings) is disabled, and you'll be all set. Why this only manifests itself when the router is in PPPoE mode isn't clear, but I guess it is what it is.

Since I wasn't able to find anyone else experiencing and later solving this issue, I hope anyone who does finds this useful.

Here's a screenshot of the WRT110 configured with QoS disabled, enabling full speed on the DSL link: