A new web app aims at offering a rich user experience.

As a functional tester, you should open a defect report providing detailed information on which devices and by running which tests you observed the issue. A defect report is a document that records the occurrence, nature, and status of a defect detected during testing, and provides information for further investigation and resolution. A defectreport should include relevant information such as the defect summary, the defect description, the defect severity, the defect priority, the defect status, the defect origin, the defect category, the defect reproduction steps, the defect screenshots, the defect attachments, etc. Opening a defect report is a good practice for any tester who finds a defect in the software system, regardless of the type or level of testing performed. The other options are not recommended, because:

The issue is related to performance efficiency, not functionality, but that does not mean that as a functional tester, you should not open any defect report as all the functional tests passed. Performance efficiency is a quality characteristic that measures how well the software system performs its functions under stated conditions, such as the response time, the resource utilization, the throughput, etc. Performance efficiency is an important aspect of the user experience, especially for web applications that run on different devices and networks. Even if the functional tests passed, meaning that the software system met the functional requirements, the performance issue observed on some devices could still affect the user satisfaction, the usability, the reliability, and the security of the software system. Therefore, as a functional tester, you have the responsibility to report the performance issue as a defect, and provide as much information as possible to help the developers or the performance testers to investigate and resolve it.