When a view has millions of elements, scrolling with the scrollbar is fast, however, scrolling by using up/down arrow is slow.

This stackoverflow post is a very good summary of the bug: https://stackoverflow.com/questions/32449642/qlistview-with-millions-of-items-slow-with-keyboard

This stackoverflow answer analyses the problem: https://stackoverflow.com/a/50904999/3336423

The slow behavior was introduced by the fix of QTBUG-21804