Of all of these lines from the poem "After Great Pain, A Formal Feeling Comes," the one in which punctuation most fully leads to a feeling that time is passing slowly is the following, the last line of the poem:

First — Chill — then Stupor — then the letting go —

This is because the line uses four em-dashes. Em-dashes are strong stops. Visually, they take up space and cause the eye to pause. For this reason, we tend to read the last line very slowly, stopping almost at each word, though the speed picks up a bit at the end. As it is, our eyes stop on "First," then "Chill," then "Stupor" (sliding over the "then"), then move more rapidly across the four words "then the letting go," as if, when the thaw comes, our emotions unfreeze and slip along more quickly.

This is a poem about the stunned, frozen emotional feeling that can follow a profoundly painful experience, such as the loss of a beloved—or any kind of deep loss. The poem describes a person in such pain that he or she shuts down entirely and finds it impossible to feel. This is described with such imagery as "the Hour of Lead," "Freezing," "Chill," and "Stupor." The capitalization of these words also slows us down—it is as if Dickinson is deliberately making them feel heavier to emphasize how weighed down with pain her speaker feels.