Maybe to use microG Framework instead of OpenGapps,
and/or Amplify module for Xposed Framework to cut-down some services/processes/apps.
Or you need to find which system and user apps got wakelock (that keeps CPU high).
@BobCrabtree The answer is in the article: you need a third party app.
"... the Mi Fit app is the link between your phone and the watch. The app shows an activity log, last heart rate measurement courtesy of the PPG sensor (note: heart rate is measured periodically or continuously during activity, but not continuously otherwise – more on that later), ..."
"So what does the third party app support look like?
... I’m currently using ‘Notify and Fitness for Amazfit’ which set me back a couple of quid from the Play Store (money well spent!) and the increased value it’s giving me from the Bip is substantial.
The key features of Notify and Fitness for me (and believe me, there are a lot!) are continuous heart rate tracking, ..."