Richard asks a very interesting question: How fast is the speed of (conscious) thought? The answer to that question is not easy, as there are all sorts of types of information in consciousness. Visual information, for instance, makes its way to consciousness in a very short amount of time (on the order of tens of milliseconds), and transitions as rapidly as we can take new information in. Affective information gets there pretty quickly, too. Limits on the speed of perceptual and affective consciousness are largely due to the limits on transmission and processing speeds. Some types of information (e.g., shapes, edges, etc.) are processed very early on, and very automatically, and are thus processed rapidly. Others, such as complex scenes and perceptions that require top-down influences, are a bit slower. As a general rule, effortful processing, is slower than automatic processing.
But what Richard is really interested is in the influence of inner dialogue on the speed of more deliberate conscious thought. Much of our more conceptual (as opposed to perceptual) conscious thought is in verbal form. In fact, there is evidence that information that is difficult to verbalize (like, say, non-linear, quadratic categorization rules) is not processed consciously*. So, verbalization is important for consciousness, and if much of our conscious thought is processed verbally, then it stands to reason that the speed of verbalization may limit the speed of non-perceptual consciousness.
But, speech itself is primarily processed in perceptual and motor areas of the brain, and at least at the phonetic and syntactic levels, is processed automatically, so it moves very fast. Internal speech moves along at about 4-6 Hz, which, as far as processing goes, isn't the fastest we can go, but it's far from the slowest. The main limit on this speed is motor. As decades of research on verbal working memory (and more recently, a great deal of brain imaging research) has shown, internal verbal information is processed by most of the same regions that process spoken verbal information, including the motor areas. The only real difference between spoken and internal verbal processing is where the information from the motor system gets sent.
So, once we're above the limits of the speed of speech, what really limits the speed of conscious thought is how effortful the processing is, and the extent of our cognitive load, two things that are closely related. In other words, really complex information will slow thought down, and so will consciously processing a lot of information. When I'm thinking about simple things, or talking to myself without consciously deliberating about content, I can think at about the speed of speech, but when I'm reasoning about a complex philosophical problem, or I'm consciously thinking about a lot of things at once, everything slows to a crawl.
* Ironically, while this information is more difficult to learn, once it is learned, we process it automaticlaly, so it's actually processed faster than the simpler, consciously processed information.