Defined by the Paint Timing API available in Chrome M60+
“First Paint reports the time when the browser first rendered after navigation. This excludes the default background paint, but includes non-default background paint. This is the first key moment developers care about in page load – when the browser has started to render the page.”
The First Paint metric answers the key use question: "Is this website loading?". First Paint times have a direct impact on Bail ratios - which is the percentage of users who abandon you site before the any core content is loaded.