Yeah, I know.
What you get, though - partly through the exercise and partly through the time it takes to make notes on each - is a better sense of the period. It's hugely useful. And yep, the occasional word might make its way into a story, but better and handier than that, the feeling for the history is magnified. As someone who's written one seventeenth century-set novel (and with others on the horizon) any tactic that gets you back to the past is to be welcomed.
By way of example, a few snippets from the early pages. These are my notes but the work derives from Green's Dictionary of Slang - a book that's pretty much essential for anyone with a love of words. Mine's an old edition picked up in a branch of The Works, though there's a three-volume set from 2010 that's a thing of beauty.
an admiral of the narrow seas: a drunk who vomits over their neighbour at table
an affidavit man: a professional witness - someone who will swear to anything
allicholy: melancholy through drink
angler: a thief who uses a hook and a pole
arse-worm: a small person
badger-legged: having one leg shorter than the other
a barleybun gentleman: a rich man who chooses to live poorly
barnacle: a toady, a hanger-on
basket-scrambler: someone who lives on charity
bawcock: a fine fellow
beanbelly: someone from Leicestershire
beau trap: either a fixed card game, or else a badly-laid paving stone, the kind that would splash one's finery
bingo: brandy (a bingo-boy is a man who loves brandy, and a bingo-mort a woman likewise)
Halfway through the Bs and you get a feel for a world obsessed with sex, food, vomiting, illness, thievery, swindles and cons, insults, mockery, taking the mickey out of rustics, and so on. Maybe not that much has changed in the last 350 years or so...