Having ushers doesn't solve the problem when the fundamental bottlenecks and crowd control techniques are not employed.
The major reason why it takes so long to board isn't so much because there are too many people. It's how people line up and get into the train. First of all, mark the floors with lines showing where the doors will open. The trains are supposed to stop at around the same spot when they pull into the station. I recall there are signs underneath the platform that the driver will see mark where to stop. Once the lines are shown, people will line up orderly and board far quicker. Now, a whole mass of people rush along the platform edge to the nearest door, creating much chaos and delays.
Once people start lining up, exiting passengers will not have to fight through a big glob of humanity to leave the train, and incoming passengers can quickly board. Once the lines get too long, people will automatically move to the back of the train, which is where the ushers can come in to help.
It would also help if the trains are interconnected as well. Then the crowds can even themselves out within as it moves out of the station.