There is a common misconception that test driven development is slower than development without tests. I've been doing this job for nearly 20 years now and I can state with confidence that over the course of a project's lifetime this is simply not true. But we're writing more code, right? How can that not be slower than writing less code? It's simple really, the code you write is more focused and your test harness massively simplifies refactoring (an inevitability of an evolving system) and regressions are inevitable without some protection. It's actually quite easy to tell from the outside whether a project is using tests properly by the regressions you see as new versions are released. See the same bug pop up a few times? I guarantee that project either isn't using tests, or isn't using them properly. Writing tests after writing the code is a really good way of writing really bad tests.
That said, not every project needs tests. Sometimes you don't even know what it is you want to do (let's say we're working on something experimental or trying out an idea), and in this instance writing tests will almost certainly slow you down unless you adopt BDD too which allows you to define behaviour at the very highest level and gives you some focus moving downward into specifics.