a) Having 4 places for people to load trays definitely speeds things up
b) People should stack their trays [diagonally] while loading, before pushing them all onto the conveyor together#
c) Clear empty trays while waiting for yours, with an unspoken passive-aggressive stance of "don't be so selfish, you asshats"
d) Say "excuse me" in a faux-polite sarcastic tone if people get in the way or otherwise act stupidly
e) If needed, consolidate all your belongings into one tray, while waiting for the bag to put them in to come out
f) Tut & glare where necessary