Why is an AppleScript to modify Calendar event attributes so slow?
Use CalendarLib. CalendarLib is a Script Library that lets you work with Calendar events without using the Calendar app. It is very, very fast:


  CalendarLib
  
  CalendarLib is an AppleScript script library for manipulating calendar events without using Calendar.app. It requires OS X 10.9 or later, as well as the BridgePlus script library.


I do not know why Calendar is so slow with AppleScript.