There is a performance regression introduced by the fix for
4533021 Container needs API to change Z-ordering of children
For more information please refer to
This is not a functional bug, but should be considered a performance tweak instead.
The current impelementation of the Z-order changes is quite expensive. Any addition (or removal) of a component to a container does restack all components of the container. The fix is to change only the Z-order of the particular component upon addition (or removal) of the component.
It's a rather risky fix for 6u14 b04.