By default, SmartCite will auto-update your bibliography as you add references. This can slow down SmartCite's performance if you have a large number of references. Many users choose to turn off the auto-update function and manually update when needed.
To do this, open the ReadCube desktop and click on the Preferences button in the bottom left corner of the app (the gear icon). Proceed to SmartCite tab and uncheck "Automatically update document after citation insertion".