The analyses of a text #2

The narrator recalls his adolescence.
He is now famous writer.
Traddles is his best frined and a lawyer, and Sophy is his able wife; she calls her husband "Tom."
Chapter 61 I AM SHOWN TWO INTERESTING PENITENTS
.................................
I wondered, at first, why I so often found Sophy writing in a copy-book; and why she always shut it up when I appeared, and hurried it into the table-drawer. But the secret soon came out. One day, Traddles (who had just come home through the drizzling sleet from Court) took a paper out of his desk, and asked me what I thought of that handwriting?
'Oh, DON'T, Tom!' cried Sophy, who was warming his slippers before the fire.
'My dear,' returned Tom, in a delighted state, 'why not? What do you say to that writing, Copperfield?'
'It's extraordinarily legal and formal,' said I. 'I don't think I ever saw such a stiff hand.'
'Not like a lady's hand, is it?' said Traddles.
'A lady's!' I repeated. 'Bricks and mortar are more like a lady's hand!'
Traddles broke into a rapturous laugh, and informed me that it was Sophy's writing; that Sophy had vowed and declared he would need a copying-clerk soon, and she would be that clerk; that she had acquired this hand from a pattern; and that she could throw off - I forget how many folios an hour. Sophy was very much confused by my being told all this, and said that when 'Tom' was made a judge he wouldn't be so ready to proclaim it. Which 'Tom' denied; averring that he should always be equally proud of it, under all circumstances.
[David Copperfield by Charles Dickens]
1. I'd like to know the object of "throw off" is the nominal phrase "I forget how many folios an hour."
2. And I'd like to know what "it" refers to.
Thank you in advance for your help.