With reference bugzilla 647074:

When at-spi2-core's event listener machinery is used from within gnome-shell to monitor focus and caret movement events, AND when those events are emitted by gnome-shell St widgets, then the system freezes for about 5 - 10 seconds. When the UI is responsive again, the AT-SPI event object passed back has no accessible component within it.

When the same code is run standalone outside of the gnome-shell process, then there is no delay, and the component is provided by the event object.

See comment 5 of 647074 for a patch to gnome-shell that shows the problem.  Apply the patch and re-launch gnome-shell from a Terminal.  Open the gnome-shell debugger (alt-f2, 'lg <return>'), and enter these commands:

1. ft = Main.focusCaretTracker;
2. FT = Main.FocusCaretTracker;
3. ft.startTrackingFocus();
4. ft.connect('focus-changed', FT.onFocus);
5. <ESC> to leave the gnome-shell debugger.

Tabbing among GTK widgets (e.g., within FireFox) will show information about the focussed component within the Terminal window.  Accessing a St widget (e.g., the task switcher), will show the freeze/delay.

If the code in the patch is launched from a gjs process separate from the one that gnome-shell is using, the delay doesn't occur.