Depending on the tab content, loading of the tab page is still delayed until the tab page is visited. However, to be able to delay loading of the tab page, the tab page content is checked for the following requirements:
If the tab page does not meet all of the above requirements, the tab page content is loaded immediately when loading the tab container. The reason that loading of the tab page can not be delayed if it contains an editable widget, is because the widget might want to trigger a validation error. E.g. an editable text box that is required.
Not loading a tab page in the current implementation means that the content of the tab page will not even be parsed. That means that widgets on a tab page that are not loaded yet are not even instantiated until the tab page is visited. So if the page would be saved without the user ever visiting the tab page, the required validation would be ignored. It would also cause microflow validation feedback to end up in a dialog instead of below the widget.
On a slightly related issue, with the use of responsive layouts, you often now want to replace tab containers with group boxes. So the issue here is whether collapsed group boxes could be set to not fetch their data until they are expanded. Otherwise there could be performance implications to updating the layout style (- or not if tab pages now retreive all data initially too)