One of the factors that Google’s ranking algorithm takes into consideration is the speed in which websites load. This is becoming increasingly important, especially on mobile devices.
This is so important that Google has developed a free tool they call “PageSpeed Insights” that that you can use to analyzes your web pages to see if they follow Google’s recommendations for making a page render in under a second on a mobile network. Research has shown that any delay longer than a second will cause the user to interrupt their flow of thought, creating a poor experience. Google’s goal is to keep the user engaged with the page and deliver the optimal experience, regardless of device or type of network.
It is not easy to meet the one second time budget. Luckily for us, the whole page doesn’t have to render within this budget, instead, we must deliver and render the above the fold (ATF) content in under one second, which allows the user to begin interacting with the page as soon as possible. Then, while the user is interpreting the first page of content, the rest of the page can be delivered progressively in the background.
You can find the tool at https://developers.google.com/speed/pagespeed/insights/