UX Metrics for the Modern Web
"Visually complete" is the moment that all visual elements are painted on the screen and visible for the user. Note that visual completeness is not the same as functional completeness. I touched on why this is so in the above section. But suffice it to say, this is not a great way to measure performance.
Start Render (or Render Start)
The "start render" event is fired in the browser when something (anything!) is first painted on the screen. The paint event may be the whole page, but it could instead be a single word, single image or single pixel. That may not sound significant. After all, if the content is not there and the user can't interact, then what is the value? Keep in mind that, before "start render" fires, the user is staring at a blank white browser screen or, worse, the prior page from which the user just tried to navigate away from. From the user's perspective, "start render" is the moment that the website is clearly working properly. There is significant evidence that abandonment (bounce rate) is correlated very strongly with slow "start render timings. Arguably, "start render" is the most important metric of all.
When the browser requests the base page, that request must traverse the Internet (whether or not a CDN is in play). The request arrives then at the hosting facility, which must fetch (or assemble) the page. Then the process goes in reverse. The response must traverse the Internet again back to the device requesting the page. "First byte" is the time it takes for the first byte of the response to reach the browser. So, "first byte" is a function of twice network latency plus server latency. Other factors, like packet loss, may also impact this metric. "First byte" is transparent for your users. However, the metric is still important because it is critical path for all browser functions.