Calendar events sourced from Google Calendar, Outlook, Microsoft 365 and the likes are quickly propagated to other calendars, as they support webhooks or native notifications. This means changes to these calendars will sync to your other Google Calendar, Outlook, Microsoft 365, Apple Calendar, CalDAV, NextCloud, Radicale, etc., calendars quickly!
iCloud, Fastmail and other CalDAV calendars still update on a timer, because Apple has no way to tell a calendar tool that something changed.
Read more about what a paid plan adds.