I worked for an airline with free seating for 18 months and the rest of my 8.5 years has been with allocated seating.
All things being equal (pax load, boarding doors used and bridge or stairs) that free seating makes boarding 30% quicker. There is always the people who want to sit closer to the front. But there is also a number of people who prefer centre or aft cabin. And people who want to be seating in the less populated part of the cabin and so forth.
I also found that because it was free seating families with children were almost always at the gate and ready to board. And you are definitely waiting less time for no shows, less often too.
As people in groups of three or more wanted to sit together they would immediately endeavor to satisfy that requirement over worrying about where on the aircraft they were once the aircraft started to fill up - meaning they were seated quicker. In my experience this also meant less people ended up separated on full flights. So you were rarely trying swap pax around. Also there was seemingly less pressure to accommodate the request than when they check in an hour before the flight and received separated seats. If they are separated (on a free seating flight) it is because they are among the last to board. Hence they were among the last to join the queue - a factor they have relatively more control over than how many people check in and receive seat allocation before them.
In my opinion it is more efficient on short flights to destinations that are not solely (or more than 60-70%) leisure. This is why you see (or saw) free seating on many full service airlines on their 'shuttle' type routes (Delta and Finnair spring to mind).