For about the 10th time .. I will post this link, as it is the expected behavior of the HRM .. until Apple changes it. It clearly and unambiguously states readings are taken every 10 minutes.
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
But I guess the response you got isn't surprising as most who buy the AW are probably more interested in Digital touch and Emojis, and don't care so much about the fitness features. You can almost see that by how inactive these threads are for the most part.
So far my early experience with the AW has somewhat soured me to Apple (no pun intended). So much so that if I return the watch, I might just quit the whole Apple ecosystem (iPhone and iPad) and look elsewhere.
i will give them until the next update to straighten this out. One silver lining is if they give full sensor access to the 3rd party developers quickly, then this will likely be moot. I don't expect that though.