It might slow one down a bit if one tries to read explanations of plans and ideas while a game is in progress.
Starting Out v. Move by Move

Generally speaking, which of these is best to read first and why?
More specifically, I want to learn the Slav and am considering the following. Of course maybe there are better options of which I am not aware.
The Slav: Move by Move by Cyrus Lakdawala (Aug 16 2011) Paperback
By Glenn Flear Starting Out: Slav & Semi-Slav (Starting Out - Everyman Chess) (1st First Edition) [Paperback] Paperback – July 17, 2005
EDIT:
Another option:
[ Play the Slav ] By Vigus, James ( Author ) [ 2008 ) [ Paperback ]Jul 1, 2008
by James Vigus
Actually, Lakdawala's older books (2011 is when he started writing his first ones) don't have so much of that meaningless drivel. I would buy it if I were you...
Forget learning lines before you play them, learn them as you play them. Use a book as you play on chess.com and you'll never forget the book line you won with again.
I doubt Its against the rules, that's why you can use opening explorer while you play too!