The font-display property defines how font files are loaded and displayed by the browser. Swap instructs the browser to use the fallback font to display the text until the custom font has fully downloaded to avoid “flash of invisible text” (FOIT).
If you enable this option, Swift Performance will set font-display property to swap for all fonts.
Of course as everywhere in Swift Performance you can add exceptions, if you would like to exclude some fonts. You can add font-family names here as an exception, eg: Roboto