I noticed the same thing when using that app and it bothers me as it makes the UI look somewhat inconsistent.

I would guess it's a limitation of the system in a way since it needs to change the page that appears if it's not the one that are "under" the app when swiping away. Hence it feels like lag while you are actuallt "cheating" the system in a way. If the process went faster I guess you would not notice it as much.