Keeper.sh now gives every source calendar its own due time for its next read. Before this release, calendars were read in shared passes, and accounts that kept failing could take up the capacity that working calendars needed.
Each read now sets the next one. A calendar that reads successfully is due again after its plan's interval, and one that fails backs off and is retried later without holding up anything else. A change notification from Google or Outlook, a newly connected calendar and a reconnected account are all read at once.
The plan timings are unchanged: on Free, your calendars are checked every minute and changes are copied every 30 minutes, and on Pro, Google and Outlook changes land within seconds and iCloud, Fastmail and other CalDAV calendars update every minute.
An event has not arrived in the other calendar covers what to check when a copy is late.