My understanding is that the computed value for 'transform' that is set for the ::view-transition-group selector includes the current scroll position of the Document (as required to place it in the current visual position if the view-transition is started when scrolled down).

This means that the -ua-view-transition-group-anim-transitionName 'from' keyframe includes the initial scroll position, and the (implicit) 'to' keyframe includes the current scroll position (and changes each frame if scrolling happens).

This has the visual effect that scrolling lags behind the movement of the scrollbar (and feels heavy), since the pseudo elements animate to the new scroll position. Obviously this is more pronounced when using a longer transition duration.

Assuming that this is the intended behaviour, it might be worth adding a non-normative explainer so that it's more obvious that this will happen.