No idea. See Use Apple TV to play audio throughout your home - Apple Support and the section Calibrate audio between your wired and wireless speakers:
If your Apple TV is connected to a home theatre receiver or TV using an HDMI cable, you may experience audio latency, a delay that sounds like an echo when playing audio on both receiver speakers and a wireless speaker.
You can use an iPhone to calibrate the audio signal on your Apple TV so that all speakers play in sync.
- Open Settings on Apple TV.
- Go to Video and Audio, then scroll down to Calibration.
- Select Wireless Audio Sync, then follow the onscreen instructions for using your iPhone to sync your speakers.
For more information, see Adjust video and audio settings on Apple TV.
Going through this process may perhaps correct any lag that you've heard about.
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