In this post, I want to look at ways to help mitigate and work around [the fact that site-speed is nondeterministic & most metrics are not atomic]. We’ll be looking mostly at the latter scenario, but the same principles will help us with the former. However, in a sentence:

Performance metrics are surprisingly challenging to get right.

"Accurately measuring layout on the web is hard. There’s no perfect metric to capture style and layout – or indeed, rendering – even though all three can impact the user experience just as much as JavaScript.

However, it’s important to understand how the HTML5 event loop works, and to place performance marks at the appropriate points in the component rendering lifecycle."