Why is the "ItemAppearing" event called for all ListView items when there are only a few visible?

Hello!

I got a grouped ListView with two groups and four item templates. They all have a few bindings. When loading the ListView it takes some time to render the rows, about 5 seconds... for 25 rows. In each row at the ViewModel i do a few calculations, for example calculating the week number of a specific DateTime and formate some lists. These calculations are heavy for all rows together.

Then i came across the "ItemAppearing" event. But this event is called for all rows, also for the rows that are not visible. I'm searching for a way to do heavy row tasks only when the row becomes visible, for example when the user swipes through the list.

Why is "ItemAppearing" called for all ListView items? Is this normal behaviour?