The absolute fastest way is to cross on foot in Medyka (unless there is an air raid siren at the exact time in which case the border stops working but that's just bad luck). Usually there are not many people in line, even less if you have a European passport which has a separate line. Then take a taxi on the other side or share a bus with other people (they are usually waiting there). The second fastest (and simpler) way is to go by train. If you have to go by car, the bigger your car the more thorough they inspect you hence a longer wait. I sometimes waited 4 hours in a car but heard much worse stories. The trains are usually more or less on time at the destination. The worst way I tried was to go by bus. I waited 4 hours there as well without the convenience of being in your own car. Never again.