Ten years ago the network was the biggest problem when it came to making websites fast. Today, CPU is the main concern. This happened because networks got faster while JavaScript moved in the other direction growing 3x in size in the last six years. This growth is important because JavaScript consumes more CPU than all other browser activities combined. While JavaScript and other activities block the CPU, the browser can't respond to user input creating the sensation of a slow, jittery, or broken page, AKA "jank".
To help focus our attention on CPU, several new performance metrics have been defined and evangelized over the last year or three. In this post I'm going to focus on these:
Here's a figure to help visualize these metrics.
Over the winter holiday we added a bunch of new metrics to LUX:
We're excited to announce the availability of the First Input Delay metric as part of LUX, SpeedCurve's RUM product.