Apple TV mirroring performance
I'm having quite the same issue not with an apple tv but with another stuff provided by my Internet broadband which is doing air mirroring. 
I have a mid 2013 macbook air and mirroring my screen is lagy. 

So I thought the issue was from the router too, but my brother with his latest MacBook Pro doesn't have any lag, it's really fluid. 

So I thought the MBA wasn't power enough too handle the real time encoding, but one day while I was mirroring my screen I launched a slide show in iphoto and it was really fluid. Then I noticed when I turn air mirroring on it's always lagy until I launch a video from QuickTime.

So, to sum up - this likely is a software issue since under iPhoto and Quicktime, the MBA is fast enough to handle high performance mirroring of some content. (Or that it lags when it has to both assemble the image to draw on the screen and then mirror it - but when QuickTime or iPhoto take over the entire screen - mirroring can be quite snappy).