SLF here - I've posted recently about this in the SLF forum, where I was griping about the strangely inept performance of SWISS cabin crew in getting everyone on board and seated. In contrast, I've witnessed some absolute marvels, such as SQ crew getting a packed 777-200 fully boarded in Jakarta in just under 15 minutes.
Having watched this carefully for some time, I would say the key factors are:
1. Active involvement from the cabin crew. If you're allowed to help stow hand luggage, that makes a big difference. But so does active, cheerful chivvying, guidance and encouragement. (The SWISS crews I was complaining about just stand around, and sometimes actively get in the way. At the other end of the scale, EZY crews work like slaves to get everyone seated, rearrange things in the overhead bins to make space, etc etc.)
2. Anticipating, identifying and quickly responding to the problems that cause bottlenecks. This is where the SQ crew really shone - they'd instantly and magically appear at the side of anyone blocking the aisle, and sort out whatever the problem was. Sometimes it's just a matter of tactfully asking the pax to stand in front of the seat while he sorts out his reading matter, or fiddles with his mobile phone. This is obviously easier to do in a twin-aisle, as you have more chance of reaching the problem area quickly.
Thanks for asking the question. Slow embarkation is something that drives me nuts, and often seems to delay the departure of flights that would otherwise be on time. I have no idea why some airlines tolerate it - it must cost them a fortune.