In the final stanza of the prologue to the Rhyme of the Dutchess May, Elizabeth Barrett Browning explains exactly what she is doing with the rhymic interlude in the midst of each stanza. This is a longish poem, and the short line, "Toll slowly," disrupts the otherwise standard rhythm of all 112 verses.
The scene is that she is reading a very sad tale, a tale of death, in a churchyard, while the church bell continuously tolls for a funeral. As Browning explains in the prologue, "The solemn knell fell in with the tale of life and sin, like a rhythmic fate sublime."
So, as we read the whole poem, and as we continually hear the words "Toll slowly" in the middle of each stanza, we hear the tolling of the funeral bell. The funeral sounds permeate the poem, "like a rhythmic fate sublime."
The Cambridge Edition of Elizabeth Barrett Browning's poems contains the following note right before the poem: "Despite the irritating iteration of the refrain Toll slowly--which most people omit in reading--the 'Rhyme of the Duchess May' has generally been accounted much the best of Mrs. Browning's longer ballads." Iritating iteration? Yes! But it is meant to be irritating, it is the constant breaking in of death on our thoughts as we read, it is death refusing to be ignored. It is the tolling of the inevitable toward which the poem is rushing us. Thus the word "slowly" doesn't only describe the bell, it pleads with it: "Please toll slowly, don't rush us to the funeral." The words must not be omitted. Or at the very least they could perhaps be replaced with the slow tolling of a churchbell. But that would be quite as irritating.
I will post only the prologue, though the whole poem is intensely beautiful.
Prologue to RHYME OF THE DUCHESS MAY
Elizabeth Barrett Browning
To the belfry, one by one, went the ringers from the sun,
And the oldest ringer said, 'Ours is music for the dead
When the rebecks are all done.'
Six abeles i' the chruchyard grow on the north side in a row,
And the shadows of their tops rock across the little slopes
Of the grassy graves below.
On the south side and the west a small river runs in haste,
And, between the river flowing and the fair green trees a-growing,
Do the dead lie at their rest.
On the east I sate that day, up against a willow gray:
Through the rain of willow-branches I could see the low hill-ranges
And the river on its way.
There I sate beneath that tree, and the bell tolled solemnly,
While the trees' and river's voices flowed between the solemn noises,--
Yet death seemed more loud to me.
There I read this ancient rhyme while the bell did all the time
And the solemn knell fell in with the tale of life and sin,
Like a rhythmic fate sublime.