What are the additional round trips?

[Ricardo Xalma]

PSI calculates that this page needs X additional round-trip cycles to load resources that block rendering and X.2 MB to render completely. The medium page needs X round trips and X.7 MB. If fewer round trips and fewer bytes are used, the pages are faster.

What are the cycles?

How are the cycles calculated or does it affect each of the cycles?

and what is a rendering?

I think that only the person who has written this text knows what the hell is talking about or is it about the cycles of a washing machine?

[Ian Turner]

A round trip is the time it takes between when a browser (client) sends a request and when it gets a response from the server. This is important because the browser can't start to render the page until it gets a response from the server.

The key thing is to keep everything on your site as small as possible, because every asset you put on your page (images, JS, fonts, styling) takes time to send and for the browser to parse (read) and render (draw).

You need to minimize the render-blocking assets (all the stuff that has to be loaded on the browser before it will even think about trying to render your page), so that it doesn't take multiple round trips before it can start rendering your page. That's what the "cycles" mean. You page has too much content to download before the browser can start rendering, so it is taking multiple round trips (cycles) to get it all.

Good luck!
-Ian

[Michael Crenshaw]

What does "additional" mean in this context? Does it mean, "in addition to the round-trip required to get the HTML"?

What contributes to the round-trip count? For example, if my HTML doc is > 14kb, will PSI count an additional round-trip because of the ACK sent after the TCP slow start?

What if I have 3 resources coming from the same CDN? Will each count as a round-trip (request resource, receive resource), or will they be treated as only one "additional round-trip" since they are being retrieved on a single connection?

Will TLS-related round-trips contribute to the count? For example, will an OCSP revocation check with a certificate authority contribute to the count if I don't use OCSP stapling?

[Rick Steinwand]

Maybe this will help: https://stackoverflow.com/questions/48478801/what-are-the-round-trips-google-mentions-in-the-pagespeed-insights-tool

The round trips PSI is interested in is to render the top of the page (Viewport).

So if you're loading a bunch of scripts, stylesheets, fonts in the head of your page, loading them as deferred or moving them to the bottom of the page (before the end body tag), may help.

A round trip is considered to be about 14k compressed. If the file is more than that, it'll take multiple trips. So every external file will be counted.

Possibly if you have HTTP/2, multiple small files from the same site, might be considered part of one round trip. I haven't looked at HTTP/2 since it first came out.