What is fully loaded time on GTmetrix and why is it important?

Question

I cleared my cache and cookies, reload a website and it loads up on my screen in less than a second, yet when I go to GTmetrix.com and test the example site, the 'Fully Loaded Time' is much longer than what I experience on my computer, even on the highest unthrottled broadband setting.

On the GTmetrix site, it says the 'Fully Loaded Time' is "the point after the Onload event fires and there has been no network activity for 2 seconds." yet on some well optimized sites, I see the Fully loaded Time as being under 2 seconds. How can it be under 2 seconds when it has to calculate the point at which there's no network activity for 2 seconds.

Also, I went to the w3schools.com definition of the download event and it says it is an event that is programmed to occur when objects such as images/scripts files/css files/etc. are fully loaded. So I'm assuming the Fully Loaded Time is when everything besides the images/js/css files have completed loading and all the extra stuff after that finishes loading as well.

For the average user, is then 'Fully Loaded Time' from GTmetrix not much of a concern since most of the website information loads quickly unless it is some sort of web app that needs to have its programmatic functions load fully as well?

How important is the 'Fully Loaded Time' metric and could you give me a use case example where this metric would be important for the page to load completely? For example, I go to Amazon.com and everything loads in under a second and I can begin shopping right away yet on GTmetrix it says the Fully Loaded Time is 14.7 seconds and even the video screen capture on GTmetrix shows the page loading rather slowly tho the images and site structure seems to finish loading by the halfway mark.

I'm trying to understand the glossary terms of page load speeds better and this is confusing me. Thanks.

Answer 1

"the point after the Onload event fires and there has been no network activity for 2 seconds."

So what it does is load the resources on the page and listen out for no more resources being loaded for 2 seconds.

At that point it works out the last time it loaded a resource and gives you a time for load.

That is how it can be under 2 seconds, it waits for the first 2 second break and then looks at the last time it downloaded a resource, that is the time it uses for "fully loaded".

"For the average user, is then 'Fully Loaded Time' from GTmetrix not much of a concern"

Fully loaded time is not as important as first contentful paint, time to first byte etc.

It is a useful diagnostic tool to see if you are sending way too much down the wire or to identify problems with the site where non essential resources are "hanging".

Additionally it is useful to see if any lazy loading etc. is working as expected as one of the criteria that is important for mobile is trying to not send unneeded information down the wire to preserve a user's data plan. (if you send me 20Mb of off-screen images that I never see then that is a waste of my data).

It is also important for things like anchored headings etc. If a page takes 15 seconds to load when i got to "yoursite.com/page#interesting-heading" there may be a lot of "Cumulative Layout Shift" if things are still loading.

If you are trying to work out what Google etc. think are important then this answer I gave on the scoring updates to Google Page Speed Insights (same engine as GTmetrix) illustrates what they actually score on and the weightings they give each critera, which is a good insight into what your users are likely to care about / what will increase conversions / time on site.