"The labour we delight in physics pain."

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,"

he saw, though perhaps not quite distinctly, that such was the poet's
intention, only, as I have said above, he arrived at it by altering, and
I by adhering to, the poet's language. I may be allowed to add that I
came to my conclusion many years before I was asked to put my name to an
edition of Shakspeare, which interrupted one of the most valuable
friendships I ever formed.