Originally “Blink” (the shared technology used in browsers such as Chrome, Firefox, and Safari (on iOS, MacOS, Android and Windows) used fallback fonts installed on the visitors’ device if the font loading isn’t finished in 3 seconds (an arbitrary number). As of May 2018 Blink now adjusts this timeout adaptively to improve performance on slow connections.
Chrome 53, network quality estimator has been used to trigger the intervention on effectively slow networks. This intervention was enabled by default in Chrome 59. Developer tools will show a warning message when the intervention is triggered for each font.