Whenas in silks my Julia goes, Then, then, methinks, how sweetly flows (1-2)
The sequence of "whenas" and "then" tells us that this poem is interested in time. The repetition of then is a little odd, though, because we can't quite tell if it suggests hesitation or emphasis. It's as if the speaker is saying one of two things: either "only if Julia is wearing silk will I then think about how hot she is," or "whenever Julia wears silks, she catches my eye. I think it's because the silks themselves are so beautiful." In other words, he might be tripping over his words as he makes up the line on the fly, or he might be emphasizing the conditional nature of his affections for Julia.
Next, when I cast mine eyes and see (4)
The temporal sequence here ("next," "when") echoes that of the stanza that comes before it, and it begins to sound a bit formulaic. The five time words in the poem ("whenas," "then," "then," "next," and "when") make the poem seem like a sort of rehearsed performance of sorts, not a spontaneous display of emotion. Not that there's anything wrong with that, but it makes us wonder about the genuineness of the sentiment.