Disagree all you want, but every time you open the calendar it checks for updates. And if you are in airplane mode there’s a good chance that slows it down even more, as the phone attempts and fails to connect to the server. And I doubt there is a patch for this very rare issue; it doesn’t happen with any of the 4 iOS devices in my household, with any of the 12 calendars the apps display. It’s an issue with your phone; it would probably be worthwhile contacting Apple for help with it using the Get Support link at the top of this page.
What is the source of the calendar data on your phone; go to Settings/Calendar/Accounts to see which calendars your phone syncs to.