Key metrics for evaluating the performance of a GUI include:

1. User Satisfaction (CSAT): Measures overall user satisfaction.

2. Net Promoter Score (NPS): Assesses user loyalty and recommendation likelihood.

3. Task Completion Rate: Measures the percentage of users completing tasks successfully.

4. Time on Task: Records the time taken to complete tasks.

5. Error Rate: Tracks the number of errors encountered by users.

6. User Engagement: Measures user interaction and activity levels.

7. Bounce Rate: Indicates the percentage of users leaving the application quickly.

8. Click-Through Rate (CTR): Measures the percentage of users clicking on elements.

9. Conversion Rate: Tracks the percentage of users completing desired actions.

10. User Retention: Measures the percentage of users returning to the application.

11. Load Time: Measures the time taken for the GUI to load.

12. Responsiveness: Assesses the GUI's responsiveness to user interactions.

13. Accessibility Compliance: Evaluates the GUI's adherence to accessibility standards.

14. Aesthetic Appeal: Subjectively evaluates the GUI's visual appeal.

15. Learnability: Measures how easily new users can learn the GUI.

By tracking and analyzing these metrics, developers and designers can identify areas for improvement and optimize the GUI for better performance, usability, and user experience.