What type of testing should be implemented to address user experience complaints about a Visualforce page taking too long to load?

Performance Testing is the most appropriate type of testing to implement when addressing user experience complaints about a Visualforce page taking too long to load. This form of testing specifically focuses on assessing the speed, responsiveness, and stability of a system under a particular workload. It helps identify how the system behaves under various conditions and can pinpoint bottlenecks that may be causing delays in load times.

By conducting performance testing, developers can gather quantitative data regarding page load times, resource utilization, and how various elements on the page affect overall performance. This testing can also help in making adjustments to the application, optimizing code, reducing server calls, and improving database query efficiency, which ultimately enhances the user experience.

Other types of testing mentioned are less relevant in this scenario. User Acceptance Testing involves evaluating the system based on user requirements and ensuring that it meets business goals, which would not directly address load time issues. Unit Testing focuses on verifying individual components of the code, ensuring that they function as intended, but does not test performance at the system level. Regression Testing checks for issues introduced by new code or changes, rather than specifically assessing the performance aspects of a Visualforce page. Therefore, Performance Testing is the focused approach to resolve slow load issues effectively.