I think a big part is how well the FAs police overhead stowage. I always sit in the last row of First Class. On the flights where the FAs block people from using far forward bins so the people who are sitting near them have space, things generally flow smoothly.
In other cases I see people in the back of the plane if not using F bins using the first bins in Y, and then when those pax get there, there's no space and the FAs have to spend time trying to re-arrange things.
Maybe announcements in addition to only one bag in the overhead -- Please use a bin either at your row or slightly behind.