My work lap is so locked down I cannot do anything personal on it, so when I go into the office I always carry two laptops, and the personal one is an old thick heavy dinosaur; it’s got to be at least five pounds. However, with a good bag that has a (non-padded) belt and sternum strap, it is not difficult. The belt carries most of the load and my shoulders don’t hurt; they hardly feel anything.
I deliberately park in the farthest spot at the other side of campus (about a half mile, and up four flights in the garage) to get in exercise steps with the heavy pack.
It’s good exercise but I absolutely need a belt and sternum pack to do it. Wouldn’t dream of trying that with only shoulder straps.