What is the fastest language to think in? If you think in any language you're never misstaken on what word you mean even if there are many words that are spelled/pronounced the same. This causes a language with only one word meaning every possible word there is to work, in theory, if used only while thinking. I don't think I'd be able to think clearly in such a language, though. The fastest language seems, for me, to be no language. It is however hard to not think in a language.

What is the fastest language to think in?
With your question you are limiting thinking to conscious and rational thought. But human information processing has so many more aspects, some of them faster and more efficient than cogitation.

If, for example, you sit in front of the computer and have forgotten that you have put water on the stove, and suddenly you hear the noodles cooking over, you'll jump up and run to the kitchen quicker than you can form the sentences that describe what is happening and what you need to do. It is not a reflex (like flinching away from being hit), but clearly a conscious understanding, a consideration, and a decision, only not verbal. For me, parts of that process are visual (I see what I need to clean away, when I hear the water hit the hot plate), others include a body-memory projection of me moving to the kitchen, a muscular plan of action that is forming while I jump up, see what is on my path, and "pre-feel" how I will navigate it at maximum speed.

Another example is what I have often experienced when I "sleep over" a decision; what has been exemplified in Einstein dreaming of relativity theory; and what I have read about again a few days ago in a book summarizing current research on intuition: that conscious rational thought is only able to make decisions in situations with a limited complexity (the level of complexity was the focus of some research), and that highly complex decisions, involving too many aspects to "juggle" in your (conscious) mind, are made on an unconscious level, with "you" (that is consciousness) only receiving the outcome (that is intuition) of that information processing.

Language is a very recent development in evolution, and consciousness is not much older, and it is my opinion that we are misled by our self-identification with the constant chatter in our minds to believe that we are this speech, while in truth language and consciousness are just tools that are as peripheral to our core selves as our arms are to our bodies. There is this famous experiment, conducted a few years ago, that showed that the muscular impulse for a movement happens before the conscious decision for that movement is made. It shows that what we perceive as decisions happens after a behaviour has already begun. The ego, as Freud said, is not master in its own house.