An interesting thing about this is that it appears to just use window.performance.timing, which is filled in by the browser automatically. They're just grabbing

An interesting thing about this is that it appears to just use window.performance.timing, which is filled in by the browser automatically. They're just grabbing data that the browser is generating anyway and forwarding it along to themselves. On my machine, calling `JSON.stringify(window.performance.timing).length` generates 623 bytes of text. Pretty much 0 overhead here.