Yes, you must use pre-clearance if it is available for your flight. If you use it, when you get to the US you just grab your bags and walk out the door, like it was a domestic flight. Coming from LHR, you would have to go through US immigration which can be quite quick or quite slow, collect your bags, and then stand in line for US customs. How long the process takes depends on how many other international arrivals are at the same time. If you are one of the first people to immigration, it shouldn't take long. Get there at the same time as several other widebodies, and you will be in line a lot longer.