Fast thinking doesn’t use words. It’s more of a knowing. Ya just “know.” In his book, Blink, Malcolm Gladwell calls it thin slicing. Somehow, without quite consciously trying you are able to make correct conclusions about something complicated.
Calling it fast think may be a bit of a misnomer, though. It may be fast in the moment, when your brain balances and computes multitudes of variables instantly, but it may have taken years of incremental thoughts to give you that ability.