Measuring site performance benefits from a close connection with publisher business outcomes. Establishing this connection is difficult and often confounded by concurrent changes to site UX, content and product offerings.
Internally, Chrome has used browser-based experiments that modify memory use and establish causal connections between memory use and user experience. These studies are known as ablation studies, and are common in other black- or grey-box testing environments like machine learning.
This is a proposal for a low-level mechanism to support ablation experiments when measuring web site performance.