It’s very hard to describe. When I’m thinking before speaking I pause, but it’s not like there are actual words going through my head, which I then speak. It’s more like the idea and words just arrive fully formed and I express it.
I’d maybe describe it like when you ask a computer to do something complicated. It takes a couple of seconds, and then shows you the result. You aren’t conscious of the computing being done to produce the result, that just goes on invisibly and then the result is there, ready to go.
I find this whole thing fascinating. It blew my mind when I discovered that some people have an internal monologue constantly.