I think I read somewhere, that if you put any custom widget on a tabpage, lazy loading will no longer work.
I don't know where I found that piece of information, though. If I can remember I'll add it.
That is true, I also read that. Thats why we want to know how Mendix loads for example a datagrid, but also what the reason is why the lazy loading functionallity is overwritten when a custom widget is placed on a tabpage.
So that we can adjust this particullar widget.
I would raise a support call for this because this kind of knowledge only some specialist at Mendix can answer this for you. It would indeed be nice if widget could use this functionality.
You could try this widget to see if you can create some kind of workaround: https://appstore.home.mendix.com/link/app/50833/Appronto/Dataview-Loader
Because this might help you to at least already show the main tab. And it gives a visual clue that when switching to the heavy load tab it is still processing.