Well... it depends. For example in STN they go probably by the book.
In other cases it might be a bit different. The only goal for them is to get the aircraft back into the air as soon as possible. Should there be any delays, it's gonna get costly for the local airport / GH.

So lets say the aircraft arrives 20 minutes before the ETD. In that case the boarding might start the pre-boarding (boarding pass check) process 20 minutes earlier and finish it as soon as possible. Lets say there are 184 pax boarded of the 189. All the missing pax have no checked-in luggage. I'm not 100% sure they would wait for the missing 5 for the 20 minutes they now have for spare. 25 minutes turnover time leaves no space for error or waiting.