Which I was told can also have to do with military use of airspace for which domestic flights are grounded/held whereas international flights are not delayed? Only hearsay though.

But let me introduce another example: Japan (and for the sake I'm going to stick with legacy carriers NH/JL). Most people check a bag (usually 20kg, however many bags allowance). Roughly 50% have a legitimate carry on, the rest are personal item only. Boarding for a 777 in high density configuration commences no more than 20 minutes before scheduled departure, most flights are on time (within 10 min, and most delays that I experienced are due to weather/routing/taxi at the destination). Now, the reason boarding is smooth and they can board a full 777 in less than 20 minutes is that anyone who gets on the plane, immediately puts whatever belongings they are carrying into the overhead bin and sits down, not to inconvenience anyone else. No shuffling through the bag to find the favorite book you wanted to read, no slowly taking off your jackets or sorts.

Then again, like you said - if you regularly experience delays in push back regardless of boarding speed, it doesn't really matter.