system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does stress tend to decrease performance at complex tasks?

I speed solve Rubik's cubes as a hobby, and I've noticed that my solve times are consistently worse when I'm feeling anxious about my performance than when I'm relaxed and in the moment. However, I've heard stress and anxiety described as a response by the nervous system meant to heighten an organism's performance in high-stakes situations. Why would stress cause me to perform worse at this task instead of better?
gpt
The stress response evolved to improve our performance at a very specific set of tasks: fighting or running for our lives. So things like energy, blood flow, and stuff like that improve. But some things also get worse at the same time: notably for your example, rational thinking and fine motor skills. 

It's possible, with experience and practice, to reduce the bad parts.