I've been thinking about that inner voice you 'hear' as you think. Is that just what you're thinking - are you 'hearing' your thoughts? Or is it something different - perhaps only a subset of thoughts, those that are conscious? Can you have subconscious thoughts? (Of course most of our cognitive processing occurs below the conscious level, but can they properly qualify as thoughts?)
How fast can you consciously think? I seem to be limited by the speed of my tongue, which seems rather arbitrary and bizarre. Even though I don't actually use my mouth to 'talk' to myself, it still constrains me in some sense. It's as if I must employ imaginary correlates of the vocal organs in order to produce the imagined words in my head - and these imagined correlates are just as slow as the real things! I can't think a stream of words any faster than what I would be able to vocalise. Why is that? (And do others find the same thing?)
Do we really think in our spoken language, or is it just the end result? I vaguely recall reading a Heinlein novel where people were taught a much compressed language which then allowed them to think several times faster than usual. Could that really happen?
It seems pretty implausible to me. I guess it would speed up conscious thought, at least, by loosening the "vocal" constraints I described above. But I doubt it would greatly influence our underlying thought processes. I don't know much cognitive science yet, so I'm stuck with the notoriously unreliable method of introspection as my only source of evidence. But it seems to me that thoughts just kind of pop into my head. That is, most of the hard work seems to be done by my subconscious; my conscious mind just takes the credit!
I'm not even sure that all conscious thoughts occur in our spoken language. One can also think in images, for example. Further, it sometimes seems like I can have a thought that is purely semantic; a 'meaning' without any 'vehicle' (e.g. words or images) to carry it. They're not very clear, is the problem. I might have this odd 'feeling', and then nod to myself and think: "hmm, yeah, that seems right... now how do I spell that out?" So, unless I'm delusional (which I wouldn't rule out!), it looks like sometimes the thought may come first, to be 'translated' into language later.
A better example might be of that common experience where you are grasping for a word that is just out of mental 'reach'. You know the meaning of it, but you can't put it into English. You can't remember what word encapsulates the meaning you have (literally) in mind.
One last thing: have you ever noticed that, when shadowing another speaker, you hear your internal voice as being simultaneous with the actual speaker? I noticed this in my psyc lectures last week; I would imagine my inner voice 'talking' along with the lecturer, and I'd hear them both at the very same time! But it must take time for my mind to process what I hear and then reproduce it, so I think this must just be a nifty example of the difference between objective and subjective time that I've discussed before. (In short: my brain represents a later event as happening simultaneously with the earlier one. It then seems simultaneous to me, because all I have access to is the representation, not the underlying neural 'vehicle' that may have quite different temporal properties.)
If I tried to speak out loud, the illusion of simultaneity would be dispelled, of course. But why does the internal voice behave so differently? One might argue that a delay is involved in physically vocalising one's thoughts and then hearing them again. Or perhaps the brain can more easily manipulate the temporal properties of our representations of internal events? (That is, although the thoughts actually occur after hearing the speaker, we can more easily represent them as occuring simultaneously, which is why it seems to us as though they are simultaneous, when really they're not.)
Well, that was some fun speculation. Hopefully someone who actually knows something about this stuff can set things straight...
Update: Mixing Memory responds.