Just because someone has an internal monologue doesnt mean they think slow. Its just a voice in your head you can talk to. Its not how you think.
I'm not great at math, but I've trained myself to do basic calculations in my head. I need to concentrate hard and it goes very slow (I'm also a fast thinker for other stuff). I do this because it's a very useful skill in day-to-day life, so worth training.
I once read that many people basically spin up a virtual machine in their mind to do math. That is how it feels to me too. All simulation, no hardware acceleration, so very slow.
I also noticed that I need to speed up female speakers more than males. I'm not sure if this is because their cadence tends to be slower, or if it's something about me.