One of the most frustrating thing about the way my brain works (or doesn't) is how long it takes me to process things.
Simple example: Today my folks came around for lunch. As they were leaving, Mum called something from the car. I couldn't understand it, so I called back "pardon?", thinking I just hadn't heard it. But almost as soon as the "pardon" passed my lips, the words Mum had said assembled themselves in my head. I didn't need to hear it again, I just needed an extra few seconds to process it.
Think about it like a person who's just starting to learn to play a musical instrument. They see the score on the page, but have to think consciously about what note that means, and where to put their hands and what to do to make the sound. So they're going to be a bit slow and clumsy. It doesn't mean they don't know how the song's supposed to go, they just can't quite make it happen yet.
My brain's a bit like that sometimes. It's not that I'm having any trouble understanding the concept you're communicating, it's just that I literally can't understand the words. Give me a moment to turn those grunts and squeaks into language, and I'll be fine.
This is deeply annoying, because one does not always have a few extra seconds to spare. You come across as a bit dim, as a snob who gives the speaker a look up and down before deigning to reply, or a complete ditz whose mind isn't on their business.
I don't really have any answers. My processing speed drops if I'm overstimulated, but with my folks today I was utterly chilled and still managed a sudden brain-spasm.
I'm not stupid. But I can be slow.