Originally Posted by got_milk
Unfortunately with a 16/1 connection and 3-4 consistently heavy users of that bandwidth I simply cannot get by with having QoS off. Low-latency applications can be miserable to use when the connection's bogged down.
I also came from a WNDR3700v1 with OpenWrt and noticed an improvement with the RT-N66U overall with QoS on. I don't think it's having QoS off, I think it's how much better the router is.
Of course the main reason for QoS settings on a gigabit router is to overcome the internet WAN connection bandwidth bottleneck for high priority services (VoIP, video teleconferencing, critical applications, etc.). As long as your LAN side is running up to gigabit speeds and your router hardware can handle the traffic then the WAN side is where the problems with QoS happen. If you have adequate internet bandwidth with low latency on the WAN side then QoS shouldn't be needed. Forcing the router to prioritize and analyze packets when there is no internet bandwidth bottleneck to begin with only increases it's processor workload and more than likely will decrease overall performance to some measurable degree.
Another consideration is that the BCM chipset in the RT-N66U already handles the WAN to LAN traffic prioritization in the hardware along with integrated default optimized settings in the OpenWRT firmware. If the router has to expend more processing power analyzing and prioritizing the packets based on custom settings in the firmware as opposed to utilizing optimized default settings for the hardware then it is logical to assume there will be performance penalty. It may not be as noticeable with the RT-N66U since it has high quality hardware optimized for performance.
It may be that other routers experience more bottlenecks and problems using QoS due to lower performance hardware and inadequately optimized firmware. I know that I had to use QoS settings on my 100/5 cable internet connection with the WNDR-3700v2 or major problems would occur with my latency sensitive applications (VoIP, online gaming, etc.). When I replaced it with the RT-N66U not only did I not have to use QoS but with QoS off all my latency sensitive applications seemed to run better with QoS off on the RT-N66U than they ever did with QoS turned on in the WNDR-3700v2 settings. I can have multiple video steams, HD video streams, multiple wireless devices connected and large file downloads running through two other gigabit routers (using double NAT) and several gigabit switches...all happening simultaneously but at the same time I can still send regular fax over my VoIP system with no problems (when it used to not be possible) and I experience little if any noticeable lag during latency sensitive online gaming (when the lag during online gaming made it almost unplayable before).