It originally used to be 2 or 3 people loading trays at a time. I hated that as with 3 fast loading pax requiring three trays (1 bag, 1 clothes,1 laptop) stuff and one tray, all the trays would be mixed up at the other end.
Now, it appers to have reverted back to 1 person at a time.

I then to work on 1 person at a time unless instructed otherwise or the person in front of me is very slow and there are gaps on the belt to the security scanner when I hint "shall I?".