This is one of the few real issues reported by Lighthouse. Although the page loads so fast that it is not very noticeable, there's a time when the custom fonts are not loaded, and the text is invisible.
We are using fontsource and the swap strategy, but it seems it needs to be fixed. Try to fix this. Self-hosting the font is a good solution, or manually declaring the fonts because we are already self-hosting.
This is a documented issue on the fontsource repo fontsource/fontsource#726. As suspected, browsers do not support custom CSS properties on @font-face declarations.
This is one of the few real issues reported by Lighthouse. Although the page loads so fast that it is not very noticeable, there's a time when the custom fonts are not loaded, and the text is invisible.
We are using fontsource and the swap strategy, but it seems it needs to be fixed. Try to fix this. Self-hosting the font is a good solution, or manually declaring the fonts because we are already self-hosting.