What slows me down is that every month or so (maybe 2) I end up pulled away from that book to tweak the first one. This has been happening for ages, and I'm just so ready to *concentrate on one book* for more than a couple months in one go. But. . .we'll be on submission, so who knows?
This is pretty much my situation. For the past nearly two years, I've been writing Book 3 in between new drafts of Book 2, which is effectively on exclusive sub now. So, FINALLY, I'm drafting the end of Book 3. I have no illusions about being "done," but I'm so happy just to get the ideas on paper at last.
Re: being on sub, even though Book 1 sold fast, I had a long wait before we started edits — enough time to write another book (I did write, but sadly it was more like pieces of other nonfunctional books). I know someone who had to start edits right after their sale, so I guess it varies, but more waiting seems pretty common.
I don't do word count goals, not sure why. I give myself daily tasks, like drafting a particular chapter or revising to page x. Maybe because YA books are short, and chopping words can be as hard as adding them.