I have a good idea. The plane load should start at 0. When you increase it, it takes time to load fuel and passengers and cargo. You have to wait at the gate not moving, until the time is passed everything you want to load is loaded. Similar to starting engine, you have to wait a while. But there will be a button that say if you want to load immediately, similar to starting engine.
Similar to this idea. You should see people moving in the cabin when you have loaded people on the plane.
This would make the sim 10 times more realistic for expert server players (specially), many people start their push back/ taxi almost immediately when they spawn and that ruins the realism, waiting for the passengers to get up on the aircraft and pump fuel process would be something really fun.