Why does "peace [come] dropping slow" in the poem "The Lake Isle of Innisfree"?

One doesn't normally associate peace and action—be it "dropping" or anything else. Yet in this case, the line "peace comes dropping slow" indicates that, for the speaker, peace is not simply an absence of noise but something positive in its own right. It's something that makes its presence felt, as if it were dropping in from above, like a rainstorm or a snowfall. But seeing as it's peace (and not some violent weather pattern we're dealing with here), it drops only slowly, descending upon the speaker ever so softly.
It is notable that the dropping peace experienced by the speaker comes from the "veils of the morning," when all is quiet, and all is calm and relaxed. This is just what the doctor ordered: a moment of relaxation for the speaker, far away from the hustle and bustle of urban life.