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.