The fact that something is possible to measure, and may even be highly desirable and useful to expose to developers, does not meant that it can be exposed as runtime JavaScript API in the browser, due to various privacy and security constraints. The goal of this document is to explain why that is the case, and to provide guidance for what needs to be considered when making or evaluating a proposal for such API's.
Latest draft: https://w3c.github.io/perf-security-privacy
Questions, suggestions? Please open an issue or send a pull request!