In my previous post I summarized my move from Gmail to Fastmail. After making this move, I noticed from time to time my iPhone battery drained quickly, with background activity by the Calendar application by far the biggest user of battery life.
The Fastmail team took a look at what was going on and indicated that, strangely, my macOS client was frequently changing the order of calendars. This explained why I saw the phone battery drain only from time to time: when my Mac was awake.
The macOS configuration doesn’t provide a lot of insight or control into this. I am no expert on iCalendar, but this doesn’t prevent me from fashioning two naïve theories about what was going on: first, possibly the Fastmail implementation doesn’t handle ordering of shared calendars well. This theory seems unlikely since I share a calendar with my wife and her phone was unaffected. Second, possibly macOS doesn’t handle calendars with duplicate names well (I had ended up with two calendars named Birthdays).
I condensed my list of calendars significantly and no longer see the issue.