解決方法
cssに
@font-face {
font-family: 'Font Awesome 5 Free';
font-weight: 900;
src:url("https://use.fontawesome.com/releases/v5.6.3/webfonts/fa-solid-900.woff2") format("woff2");
font-display: swap;
}
@font-face {
font-family: 'Font Awesome 5 Free';
font-weight: 400;
src:url("https://use.fontawesome.com/releases/v5.6.3/webfonts/fa-regular-400.woff2") format("woff2");
font-display: swap;
}
<head>~</head>に
<link rel="preload prefetch" as="font" type="font/woff2" href="https://use.fontawesome.com/releases/v5.6.3/webfonts/fa-regular-400.woff2" crossorigin>
<link rel="preload prefetch" as="font" type="font/woff2" href="https://use.fontawesome.com/releases/v5.6.3/webfonts/fa-solid-900.woff2" crossorigin>
を追加する。