It is because Ferdinand delights in the labour that he does not feel it irksome:
"This my mean task
Would be as heavy to me as odious; but
The mistress which I serve quickens what's dead,
And makes my labours pleasure."
He, therefore, tells us, at the close, that his labours are refreshed by the sweet thoughts of her; that, in fact, his toil is no toil, and that when he is "most busy" he "least does it," and suffers least under it. The delight he takes in his "mean task" renders it none.
Such I take to be the clear meaning of the poet, though somewhat obscurely and paradoxically expressed—
"Most busy, least when I do it;"
and when Theobald proposed to substitute
"Most busy-less when I do it,"