Progressive loading web application: tutorials?

Question

Web development

Good afternoon.
Prompt materials on progressive loading of web applications and testing the speed of loading and rendering of the site.

Answer 1

It is better to load what should form the display in one thread, i.e. load from sections. Everything else (according to user events) is better to load via ajax without reloading the SPA application page.
For example, I load a page/block with information on a click in a menu item using includeHTML (loading from the "storage" sections of the current page - will be added soon).
And what needs to be downloaded immediately - I download the content on the server in the same stream, i.e., in the body of the SPA application page itself.
Download speed can be tested here: https://webpagetest.org/