Optimising Font Performance

Hi
When running “think with google” test I get the following recommended fix:-

“Ensure text remains visible during webfont load”

I have tried using default system font and default system font and adding “@font-face { font-family: Helvetica; font-display: swap; }” to additional css

I would like to use roboto if possible but my main priority would be mobile load speed.

Please can you help me to achieve this recommendation?

Many thanks
John