The study is published in the British Journal of Psychology under the sexy title, "Effects of Disfluency in Writing."
The study's authors suggest that slowing done your writing makes it less likely that you'll go with the first word that occurs to you. Instead, you have time to consider alternatives and use the best one.
Some writers have found the same effect when they do their first draft in handwriting. I've found it to be true when I've used long airplane trips for writing something by hand.
Especially if you find it difficult to reconsider your word choice when doing a second draft, slowing down when you write the first one could be useful.
In any event, it's a useful reminder that faster is not always better.