Just to give a response from the other end - I have the 38mm Sport and my heart rate is monitored on the mark every 10 minutes.
(Except for the 6-8 hours when I'm asleep and the watch is charging).
I have to sometimes force-quet Health or the Activity app on my iPhone 6 to get the new data to show up but it's definitely there.
Regarding the energy saving theory - I don't think that's the case since in the normal 10min cycle the watch will only use the
green LED when absolutely necessary and fall back to infrared reading when possible (less power hungry).
[see here if you're interested in the details: https://support.apple.com/en-us/HT204666]
One thing I noticed however is that there seems to be a *some* connection between movement and reading accuracy.
I use the original white band with the watch and it fits snuggly to my wrist, or like the user guide states it "it doesn't move but is not to tight".
I did a longer walk today on mostly flat terrain to improve my step counting and the watch did do a constant measure of my heart rate.
I noticed that two times in that 40 minutes it recorded two spikes of about 200 that lasted for about a minute...
That data seems odd as the readings before and after are pretty much my normal heart rates when walking and I didn't do anything fancy
such as running up hills or even getting the least bit exhausted.
So there seems to be at least something realted to the movement vs. reading theory...
[Also: sorry for ranting!]