I fully agree with Paul. I have the exact same issue.
Here is my analysis and test results:
Tested with following to find the root cause...
Test1:
1. Changed my router to Asus ac router (RT-87U) and getting excellent bandwidth. (50MBPS with ATT Uverse)
2. Connected Apple TV to 5GHZ channel from router.
3. Connected MacBook Pro (OSX Mavericks) to same channel on the router.
4. Switched Macbook Pro to Apple TV (Airplay display with Mirroring).
Result:
Observed lag during video streaming. Audio seems to be Ok though found few instances where there were breaks in audio signal.
Test2:
1. Connected iPhone 6 plus to again same channel on the router (5GHZ)
2. Switched streaming from iPhone to Apple TV
Result:
No logs observed and both Audio and Video streaming seems to be working OK.
Test3:
1. Switched to Macbook Pro (OSX Yosemite) and connected to Apple TV
2. Streaming video and audio test.
Result:
The video almost freezes for few mins and result is worst. Apple TV is unusable for video streaming with OSX Yosemite / Macbook pro.
notes:
1. Tested Netflix and Youtube directly from Apple TV and is working fine without any issues in HD mode.
2. Tested tv channels (ABC news) from Apple TV and no issues found using my network.
My Conclusion:
Apple TV / iPhone seems to be working fine.
Apple TV/Macbook pro doesn't work well with Video Streaming. (Periodic Lags in video signal)
The problem is not due to any issues with router or network but is clearly with streaming protocol between MacBook and Apple TV.
Clearly Apple needs to resolve this.
Hope this helps.