What is the conclusion of Bartleby the Scrivener? What is Bartleby's work performance like initially?

Near the end of Bartleby, the Scrivener, Bartleby dies in the Tombs prison, where has been sent because of his homelessness. Bartleby dies of starvation in prison because he prefers not to eat there.
Earlier, when he is first hired as a scrivener, Bartleby's work is exemplary. He is initially very hard-working and productive. Only later does he begin to say "I prefer not to" in reference to his work. "I prefer not to" then becomes his signature phrase.
At the very end of the story, the narrator hears a "rumor" that Bartleby once worked in the Dead Letter Office in Washington, where letters that for some reason could not be delivered to their proper recipients end up. The narrator ruminates on what it must have been like, day after day, to read and then burn letters that never arrived at their destination. As the narrator puts it:

Sometimes from out the folded paper the pale clerk takes a ring:—the finger it was meant for, perhaps, moulders in the grave; a bank-note sent in swiftest charity:—he whom it would relieve, nor eats nor hungers any more; pardon for those who died despairing; hope for those who died unhoping; good tidings for those who died stifled by unrelieved calamities. On errands of life, these letters speed to death.

The narrator believes that for someone like Bartleby, "prone to a pallid hopelessness," such a job could well have increased his sense of isolation and alienation, leading him in the end to shut down completely.