Loader for 4 years here. Yes, your body will be sore as
for a while. I think it was 3 months until my back stopped hurting, and 5 before my shoulders and chest stopped hurting. Those are muscles, mind you. You can build up muscle, but if you are overexerting yourself or moving inneficiently, you can end up with joint and tendon problems. Musculoskeletal problems. Avoid them.
So, how to work fast and avoid musculoskeletal problems? Well, what is the quickest distance between 2 points? A straight line. Keep your rollers close and pivot (don't twist). This will allow for more fluid movement, as well as reducing the time it takes to walk to the rollers. Plant yourself there. If you are doing more than 3 steps to go from wall to rollers, you are doing it wrong.
Grasp your packages and rollers (when moving them) evenly. Pick up all packages with two hands unless it is extremely light. You don't want carpaltunnel from grasping 1000s of 5 lb packages with one hand every day of the week. Don't favor one side or you will be off balance and risk hurting one side pretty bad. Load 100 packages on one side, 100 on another. DON'T FAVOR SIDES. You will end up with a repetetive strain injury.
If you are on the the right of the chute, work right to left. If you are on the left, work left to right. Try not to just place packages anywhere. Keep packages as level as possible when stacking so you know you can fit most packages in snug. Keep the front of the wall as flat as possible, but still balanced. Front of wall 1 to front of wall two should be about 2.5 feet, depending on the average size of your packages. Use the space between for small packages, bags, and odd shaped packages. Leave space at the top for bags and pillows and soft crap.
You can also put the smalls in front of the wall you are building as well to get a start on the next one.
If you get a "straight shot" and 40 of the same boxes come down at once, you can stack them off to the sides quickly and have the cornerstones of your next few walls already in place.
Also, your truck will get dirty and packages will be everywhere. Expect some nights to even be trapped in your trailer. Not even kidding. You can't be fast, clean, and have A+ load quality all the time.
If you can, get a truck whose chute you can see from outside while smoking.
And finally, and sadly, you will come to know the shape of your packages and know where they fit, like lego pieces.