I've normally felt "slow" not just mentally but in a broader sense. Here are some examples:
- Doing less tasks / day than most people
- Strongly attach to my interests and span for a long hours,
- Learn slower than others, sometimes due to obsession at details other times due to my own inability to do it fast,
- Walk slower, and feel quite inadequate in large cities, or busy streets.
In a way, brightness and loud noises feel a bit like "fast" to me as well, maybe it is somehow connecting noises to large cities and to "fast". I'd say even shyness and anxiety relate to this idea of speed: anxiety is sometimes my resistance to move faster; shyness too.
I wonder how it is for you.