QOS is quality of service. It lets you prioritize certain traffic over others. So if I'm running BitTorrent but trying to watch Netflix, I can set up a QOS rule to prioritize Netflix over the BitTorrent. Netflix might only need 4Mbps but a torrent can take 100% of my connection. So I would set a QOS rule to make sure that Netflix gets what it needs then the torrent can have the rest.
Inspecting every packet as it runs through the network takes a lot of processing power. Many router manufacturers put just enough hardware in the box to handle fast connections, but not enough to handle complicated QOS rules on top of that.
Looks like Google is doing the same; that CPU probably isn't fast enough to handle NAT and QoS at the speeds the WiFi radio is capable of; much less a gigabit Google Fiber connection. While I understand consumers aren't going to spend $500 on a router, it's a bit disappointing to see Google engage in the same behavior that Linksys, D-link, et al do.
Inspecting every packet as it runs through the network takes a lot of processing power. Many router manufacturers put just enough hardware in the box to handle fast connections, but not enough to handle complicated QOS rules on top of that.