It's the scale of it. "Here is a vaguely understood problem, that might take 2 hours to solve." "Here is a vaguely understood problem, that might take 2 years

It's the scale of it. "Here is a vaguely understood problem, that might take 2 hours to solve." "Here is a vaguely understood problem, that might take 2 years to solve." Though if you're used to the latter timescale, you might not consider the former as all that "vaguely" understood.