A good designer who is willing to work with a lean development team can remarkably speed up your progress. Let's look at some different ways that design can help you quickly get your product into the hands of your customers.
I Can Design Faster Than You Can CodeSo, you think you're a pretty fast coder, right? I believe you. But, unless the feature is dead simple, you can't build a working version of it faster than I can build a fake version of it. Remember, my prototype doesn't have a backend or a database. It doesn't have privacy controls. It doesn't have to support multiple users. It doesn't even have to work at all. It just has to look like it works.
Getting rid of all those requirements makes my prototype really, really fast to build. That means I can build several versions of it in the time it might take you to get the backend set up. If I make those versions seem like they work, I can get the prototype in front of users, get their feedback on it, and iterate much faster than you can.
That means, by the time you build the real version, it's already been through two or three versions and validated with customers. Many of the major usability problems will already have been found and eliminated. Complicated things will have been simplified. You'll already know how it integrates with the rest of the user interface. If you assume that you'd need to do all of these things anyway, this will save you time.
I Don't Need to Design it All UpfrontMaybe you've only worked with waterfall-style designers in the past, but there are some of us who don't need two months and a full set of specifications to get a product designed. We can get started on the broad outlines just a little before you begin coding and have something ready for you to get started with right out of the gate.
Besides, all of the products I've ever worked on had at least some non-customer facing code that needed to be written, so maybe you could write that part first. If you have some idea what you're coding based on things like customer stories (you did write customer stories, didn't you?), you can get started on anything the user doesn't see, and I can get started on everything the user does see. By the time you're done making sure the feature isn't going to bring down the servers, I can have built and tested a few mockups.