Start render optimization takes place before the first content appears to the user, and is critical for good HCI feedback. A fast start render time gives the user visual feedback
that the web server is responsive. Ideally you want a start render time (and useful content
display) of under 1-2 seconds (see Website Optimization Secrets for details). The start render (the time it takes for the first visible changes to appear) is composed of Time to First Byte (TTFB) connect time, server response time, processing objects in the HEAD of your document, and initial page parsing and rendering. Optimizng your start render time is a matter of optimizng each of these delay components.
start render time optimization
Flush HTML Early and Often – flushing html to speed up start render times and page rendering
When a user requests a page, it can take from 200 to 500ms for the backend server to create the HTML. During that time the browser is idle, waiting for data to arrive. Developers can speed up start render times and the display of useful content by flushing the buffer. Flushing HTML sends a partial HTML response to the browser, which modern browsers can display. Flushing allows the browser to start fetching components and rendering the response while the backend can continue creating the rest of the HTML page.