Putting random piles of LEGO brocks back into sets has several steps. After acquiring a mixed pile of LEGO the first step is sorting the bricks into different piles based on their shape. After identifying the potential sets the next step is collecting all the pieces for that set and listing the missing pieces. Then I need to go hunting for the missing pieces. When I finally have (hopefully) all the pieces it's time to start building.
The three most time consuming steps are the sorting of bricks, collecting the sets and building. Doing all those separately is fine. I like that kind of tasks even though it might not sound fun for most of the people.
But often, while building I find I have either missed a piece or picked a wrong one. In those cases I need to interrupt the building and get pack to my boxes of spare parts (or sorted pieces of the next lot). It's annoying! It interrupts the flow and is an extra time away from completing yet another set. I should definitely pay more attention in the set assembling phase to make sure I get all, and correct pieces in the first place.