I have two apple TVs 4th gens with speakers:
One feeds the patio speakers, and I can hear NO latency between those speakers and the kitchen HomePods when I open the window to the patio.
My other Apple TV is connected to a Sonos Playbar. I expected latency there because Sonos apparently introduces latency with the line-in. Surprisingly, when I stand in the doorway between the kitchen and the den, there is almost no latency - no echo with vocals, no differences in the beat at all. Your brain tells you it's not 100%, but it's way better than I expected. If I leave this at a reasonable volume, you'd never know there was a difference walking room to room.
No latency between the stereo pair of HomePods in the kitchen and the one in the bedroom.