Inherent speed does not matter and is ill defined, it only matters relative to the environmental processes we have to react to. We’re already orders of magnitude faster than processes like geology and weather, so it is really only other animals where speed matters, and since we’re all using the same basic hardware, we’re on a relatively level playing field there (except for house flies it seems, lol). Time as we understand it may be as much as a cultural construct as anything else (see, for example Timeless physics). Some neurodivergent people, and people from very different cultures don’t experience anything like what most people in our culture refer to as time.
As for thinking about one thing at a time- I am absolutely certain this is false. Our subconscious operates massively parallel, and we only have one conscious thought at a time. But it had an interrupt system that lets it either act instantly itself in an emergency, and to bring things to consciousness when they are important. I’m sure everyone has had the experience of reacting quickly to an emergency with no conscious thought, or suddenly knowing the solution to a problem you hadn’t even been consciously thinking about.