Measure it so you can improve it, but how?

…performance metrics are surprisingly challenging to get right.

On the one hand, it is hard to design metrics that accurately represent the user experience. On the other hand, it is difficult to make metrics that are usefully precise. As a result, many teams cannot trust their performance data.

Even with accurate and precise metrics, the data is hard to use. How do we define “fast”? How do we balance speed and consistency? How do we quickly find regressions or see the impact of optimizations?