In my opinion, the problem is not with the speed of boarding; it is with the speed of cleaning and "catering" the plane.

I've always thought that one way to speed up boarding (amongst many) would be to have the cleaners and caterers do the first class section first, then when that is done go ahead and board first class to get it out of the way.

Then, several minutes later, board the coach section once the cleaning and catering is done there as well. Instead, they wait until the entire plane is cleaned before boarding anyone at all. It makes little sense, particularly on those aircraft where boarding takes place through the middle door.