Today’s update improves the calendar’s responsiveness when fetching events and disabling calendars or navigating dates. (This update also adds the ability to assign the “Before Events Fetched” action to read-only calendars.)
The responsiveness improvement allows you to turn off a calendar while it’s fetching events. This will immediately kill the fetch process, and the UI will respond right away. The one exception is in FileMaker: if a script is actively running, the web viewer cannot receive input. So, if events are being fetched, that process will most likely need to finish before the UI can respond to input.