Yes, this is the issue described in Extension Wizard - file open dialogs hang UI for several seconds. It seems that accessibility features (that helps people with disabilities to use the OS) in MacOSX have changed in a non-backward compatible way: software that is built targeting older OS versions sometimes hangs for tens of seconds when used with new OS versions.
One possible solution would be to upgrade Qt to the latest version (hoping that Qt may be able to abstract away the OS API changes). If that does not work then we may need to increase the minimum required MacOSX version (which would prevent running Slicer on older OS versions but would make it run well on latest OS version).
Let’s continue the discussion here.