first-meaningful-paint

This metric measures the when user perceives that the primary content of the page actually becomes discernible.

FMP is when the biggest above the fold layout change is evident. The lower the score, the faster the user thinks it loaded.

Quickening the paint brush

Optimizing critical rendering path is an essential step in improving FMP: Prioritizing the visible parts of the page in relation to user interaction. Understanding how the HTML, CSS, JavaScript, etc. is being rendered, both on desktop and in mobile are vital. How the browser converts those elements and starts rendering web pages will go a long way in understanding your web performance.

Google mentions other components to improve page load time here:

• Tracking FMP using hero elements
• Assessing loading performance in real life with navigation and resource timing
• User timing marks and measures
• A layout-based approach