Comment 33 for bug 1458593

If the Offline Support option for network calendars is disabled, enabling it should improve performance. Right-click on each calendar in the calendars list on the left side of the window, click Properties, and make sure the Offline Support option is checked.

If the calendar is not shared with other users you can also set the Refresh interval to manual, that will prevent the calendars from re-loading periodically in the background.