INTJs aren't any more lazy than the other types - from what I see. ENTPs are far lazier in the true sense of the word.
I have times when I am completely lethargic, but that is because I spend much time in my own head, weighing the different options and refining my mental framework, which can greatly impact performance. This lethargy seems to be similar to the one described by the others on this thread. I focus so far into the future at times that my present situation slowly deteriorates, at which time I have to compose myself and continue along more narrow lines. It can develop into a bad habit if not controlled, and procrastination becomes a blight after initiative kicks in but no options are left. The listless state is along the lines of Sherlock Holmes; this is the best way I can describe it, except that it isn't nowhere near that extreme. If I don't have something to work towards, and see no means of getting there anytime reasonable, I begin to retreat into my own imagination.
If INTJs are ever "lazy," I believe it's because their own inner standards are more important to them than traditional measurements of "success." If one is highly introverted, it is far more difficult to act proactively in the "real" world; I can easily see how many INTJs (including myself) would turn to less 'energetic' forms of interaction, such as internet forums or chat rooms.
Lastly, I don't believe that INTJs are "underachievers," because it's contradicted by observing the real world. INTJs are definitely overrepresented in the executive world, scientific world, political world, and even the arts. It seems to me that although some INTJs are simply lazy, when INTJs does do apply themselves they reach high places.