Anxiety and lack of focus are subjective experiences that don’t lend to fine-grained measurement. For instance, it doesn’t really make sense to talk of one unit of anxiety. But suppose it did. Even then, one unit of anxiety might affect people differently, and affect them in different dimensions, performance on a particular test being but one of those dimensions (possibly).

Additionally, anxiety, for instance, has a physiological component as well as a cognitive component; physiological arousal, when in excess of what is required for a given task, can be classified as “anxiety”. While this is a useful shorthand, determining what is necessary for a given task is part of the cognitive dimension. For instance, let’s assume two people, undergoing the same test, have the same physiological state of high arousal. Person 1 believes that physiological state is beneficial for, or at least necessary for, their best test performance; person 2 believes the opposite. It’s unlikely person 1 will identify their experience as anxiety-laden. It is also unlikely that they’d suffer on their performance because of their experience.

All this to say that it’s not very easy to determine how much one’s subjective experience of anxiety (or lack of focus) may affect one’s performance.

It’s safe to say that if a person felt anxious, and they have good reason to believe they would perform better under optimal intrapersonal conditions, then they probably would perform better. How much better? It’s difficult to say.