Thing is, writers are impatient people. I'm impatient. We want to go out on submission now, we want to query agents now, we want a book deal now, now, now, now! I've had an agent since late April and we're just now preparing to go out on submission. Why so long? Distance.
I took a good three months away from my book before I went back to do a big revision. I mulled over the plot, the characters, and the overall structure, and I made some changes that entirely altered the story. It's so much better now than it was. And if I hadn't waited, if I had let my impatience rule me, I don't think I would've had the perspective to figure out what changes the story needed.
So here's my advice for unagented authors especially, since I know how tempting it is to revise quickly and send out queries. Take your time. After you get notes from a critique partner or a beta reader, spend a few weeks just thinking about them, brainstorming how you can alter your story to fix the identified problems.
Distance and perspective are key. If you're serious about a writing career, waiting a few more weeks to send out those queries shouldn't matter.