In this case, thoughts do seem to be the upper bound. If we are talking about linear streams of vocalised thought, then I suspect we might not get much further from where we are. I, for one, find it harder to stream thoughts faster than I can talk.

But consider any highly complex communication task. For instance, say you have to explain how recursion works. You have to go through abstract thinking, analogising, structuring, word selection, and delivery. It's hard work, and unless your execution is spot on, vital information maybe lost in transmission. Importantly, your thoughts here may not be a linear vocalised stream, and the process may not take very long.

It seems that tasks like these would benefit from some form of information exchange mechanism that doesn't rely on information being encoded in present day language. It would be most efficient to somehow "compress" your thought process around recursion as you're thinking about it, and then have the receiver unzip it on reception.