What Is Meant By Server Response Time

Question

I'm doing website optimisations using Google's Pagespeed Insights to test improvements. Among the high-priority fix suggestions, is this:

Reduce server response time
In our test, your server responded in 2.1 seconds.

I read the 'helpful' doc linked in this section, and now I'm really confused.

Is the server response time the DNS response, the time to first-byte, or a combination? Is it purely a server-side thing, or could this be affected by, for example, a slow JavaScript resource or ready events in the DOM?

Answer 1

My first guess would have been that it's the time taken from the moment the request was issued, to the 1st byte received from the server, however Google's definition is not quite that:

(from this page https://developers.google.com/speed/docs/insights/Server)

Server response time measures how long it takes to load the necessary HTML to begin rendering the page from your server, subtracting out the network latency between Google and your server. There may be variance from one run to the next, but the differences should not be too large. In fact, highly variable server response time may indicate an underlying performance issue.

To take 2.1 seconds would suggest to me that your application/webserver is buffering it's output, so all your server side processing is happening before it sends the content. If you don't buffer then the html can begin being sent to the browser more quickly which may help, however you lose the ability to do things like change response headers late in your logic.