ADHD isn't necessarily fast thinking, honestly, it is more accurate to call it an attention dysregulation. It might look like fast thinking because it can resemble someone with a regulated attention but who can context switch quickly. In actuality, it is more like someone who cannot control when their thoughts switch + their thoughts lose context suddenly. So it would be more like when you stand up, go to another room & forget what you went to the room to do but... everything, all the time.