The internal noise only slows when I'm drunk or on a sun holiday. First time I ever went somewhere hot for a week I literally felt my head slowing to a crawl. It was revelatory.
My internal voice is my voice. But it doesn't "speak" my thoughts as they're happening with courses of action or suggestions of what to say. It's often just chuntering away in the background, with phrases stuck in a loop, and there's ALWAYS an earworm - today it's Babs O'Reilly really fucking loud, and the phrase "He's a good finisher, for sure" over and over.
More it's a never slowing succession of connections, like visual "flashes" of images coming very fast, and times of stress or in a rush I can vocalise a lot of these connections to try and slow it down.
I'm good at concentrating for short periods and can "background" this noise when talking to people sufficiently that they would never know, but sometimes in a lull in a conversation I can jump a number of steps ahead without realising the other participant(s) have.
It's exhausting, often.