Bike makers push their bikes’ aero credentials, especially on more expensive machines, with claims of wind tunnel testing and time saved. Time was, an aero frameset was significantly heavier than one with the traditional round tubes, but the best aero bikes can now be as light as a non-aero one.
On the other hand, around 80% of the wind resistance comes from a rider, not the bike and those time savings are typically when riding at around 45kph/28mph. Since wind resistance increases as the cube of speed, if you’re riding at half that, you’ll have an eighth of the drag, so all those aero features won’t make a lot of difference.