Negative emotions impair your performance because their HRV is too high

Your Heart Rate Variability is the one that affects your performance, not your Heart Rate. You can perform well both with a high HR and a low HR, as long as your HRV is optimal. However, if your HRV gets too high, your heart rate is erratic and your performance suffers.

Your HRV depends on the emotional state you're in. Positive emotions such as excitement and contentment have an optimal HRV and thus enhance your performance, whereas negative emotions like anger and apathy have an abnormally high HRV and impair your performance.