Two possible solutions -
Option 1 -
Could you click on the Basecamp menu tab in the top left - select preferences - advanced - move the slider bar approx 3/4 of the way along to allocate more disk cache space, this should improve software performance.
Option 2 -
This can be caused by a conflict of Garmin Software.
Please uninstall all Garmin Sofware on your Mac (including garmin express installer and communicator plugin for mac).
Reboot your Mac and then reinstall Garmin BaseCamp.