I am also experiencing major wifi issues (low bandwidth, randomly high latency) on my mid-2012 Retina MacBook Pro with Yosemite and my Asus RT-N66U router. Removing all saved wifi networks as previously suggested in this thread unfortunately did nothing for my issue.
Connecting with the Apple USB Ethernet adapter makes the problem go away, but more importantly disabling WPA encryption on the router resolves the problem. Obviously, this is not a long-term solution, but I think it indicates exactly where the problem lies...
Since I only have one machine on Yosemite. What I did was create a Virtual Router on one of my PC's using http://virtualrouter.codeplex.com/ . My old solution was creating a guest wifi without WPA encryption. It's starting to seem like Apple won't be fixing this bug with Asus routers.