Originally, Blink use fallback fonts if the font loading isn't finished in 3 seconds. But this timeout value wasn't defined in any spec. Blink changes this timeout adaptively to improve performance on slow connections. If font-display feature is enabled, this change happens only if 'auto' is specified to follow the font-display spec. Otherwise, it happens always. The definition of slow connection would be changed in the field trial.
Docs: https://developer.mozilla.org/en-US/docs/Web/CSS/@font-face/font-display