Bottlenecks in the brain limit our ability to multitask
Recognizing new information in your surroundings and incorporating it into your view of the world is referred to as “encoding,” while reacting to stimuli and making decisions is termed “response selection.” They’re distinct processes, but they utilize some of the same parts of the brain, so it’s possible that the brain can have trouble if asked to perform both of these processes at once or in very close succession.
I’m a singletasker, I actively avoid distractions and other stimuli which can interfere with the task at hand. I’m also very good at switching context and working on various projects but I’m careful to not start processing information from one context while another context is in focus. Sometimes you can’t control what your mind is processing so you better let it process those thoughts and take notes instead of trying to unsuccessfully concentrate on something else.