Colors were hand done, lengthening the process even more.
I can imagine an artist shuddering at the sight of the engraving made from his work, or giving a pleased sigh, depending on the skill of the man with the burin. An engraver could add a personal touch. When reproducing John James Audubon's Trumpeter Swan, someone added a butterfly. On one level, I approve of the butterfly. It almost gives it a Disney-esque touch of a relationship between the subjects and evens out the composition. Or another level, I think it was unfair to the artist unless he asked for it, and unfair to the viewer who might want to see the most exact reproduction of Audubon's work possible.
The following video is a look at engraving, and is understandable. Don't let the fact it may not be in your native language discourage you from watching it. But, since the video is a bit long at around 8 minutes, you might enjoy it better by skipping through to watch the different aspects.
Bird pictures my own.