PBW has an excellent post about writing speed and how to write faster. She says, "There is a myth popularized by the literati that fast writers write nothing but sh*t, and should be regarded as writing sluts who only do it for money." I've seen people say that authors can't be expected to do good work if they have to turn out more than a book a year. But as I commented elsewhere once, it's perfectly possible to spend a year carefully crafting a book, only to find yourself with a year's worth of carefully crafted cr*p. Writing speed doesn't have much to do with writing quality, IMHO. Writing slowly is no guarantee of quality, and I know numerous romance authors who turn out very good work at a very high speed.
I'm not sure I totally agree with PBW's implication, however, that if you focus you should always get a certain number of pages done. I'm a bit erratic (admittedly less than I used to be)-- sometimes I like to wander around in a daze for a few days and plot, rather than actually type. Sometimes I go work on typing another book at the same time, so as not to lose writing time. But other times I just need to pull back from the computer for a few days, so my subconscious can churn out ideas. That's how I work, and I'm not sure I can get around that by forcing myself to write. I don't want my subconscious well to run dry. I'm not saying that PBW is wrong, mind you (the woman turns out great books at an amazing rate!) but simply that what works for one writer doesn't necessarily work for another.
That being said, though, I'm sure we could all write better, and faster, with more focus and concentration. So... go read her article and take from it what you will.