Users expect fast-loading and smooth web experiences, so developers should focus on achieving these goals. Web performance is influenced by latency and the single-threaded nature of browsers. Latency, the time it takes to transmit data, is a major threat to fast-loading pages. Developers should aim to make sites load quickly and ensure smooth interactions. Navigation involves entering a URL, clicking a link, or submitting a form, and minimizing the time it takes is important. DNS lookups, TCP handshakes, and TLS negotiation all add to the time it takes to load a page. The browser parses HTML and CSS, builds the DOM and CSSOM trees, and then renders the page by applying styles and layout. Painting and compositing are the final steps in the process.
https://developer.mozilla.org/en-US/docs/Web/Performance/How_browsers_work