hm, I think there are many sides to this coin.
Quality and experience is gained through completing projects and pushing yourself. Pushing your self can mean either through achieving an art quality level or completing within a strict time frame.
I don't think there's a true right answer to this question and it's merely up to each artist to answer for themselves.
There are artists like Doug Tennapel that thrive with placing intense time restrictions on themselves (like for Doug, completing 1 graphic novel a year). And under that he created GREAT book like Cardboard and Creature Tech, and even awesome webcomic series like Nnewts and RatFist.
And then you have an artist like Jeff Smith who dedicated decades on completing his fantastic series Bone.
Great things can come from either choice. However, the longer I make comics (about 5 yrs now) the more I think, I only have a limited amount of time in my life, perhaps not getting too wrapped up in the details and pushing myself to create faster and focusing on telling as many fun fantastic stories as I can is the best way to go. Let my quality come with age and experience after completing a library of books. X3