If the pipe is installed in or under the slab, the odds are pretty good the temperature drop is simply due to the heat loss to the slab or soil. Neither concrete, soil, nor copper is very insulating, and running 120F water at 1-2gpm through half-inch pipe in a concrete slab is pretty much how radiant floor heating works. If any of that slab extends out beyond the wall insulation, it's going to be a bigger issue in mid-winter than during the summer
Running the potable plumbing under the slab a common (but pretty crummy) way to route plumbing in slab on grade, bad for hot water heating/distirbution efficiency, and impossible to repair. But if that's what you've got, it's probably not cost effective to do anything more than bump the storage temp up. In some areas builders are required to insulate any buried hot water distribution plumbing to avoid this problem, and the efficiency hit associated with it. But even there it can still happen if the inspectors didn't see it before the slab was poured, or if it wasn't clear after the pour which was intended to be the cold side plumbing and which was the hot.