Current implementation is already the best one.
However, if you wish make changes
@font-face{font-family:'FontAwesome';font-display:swap;src:url('../fonts/fontawesome-webfont.eot?v=4.6.3');.....You just have to add font-display: swap; exactly like above in line 4 in
iconic-one-pro -> fonts -> font-awesome.min.css
This will fix the error in PageSpeed but you will have squares before icons are loaded because CSS property font-display: swap; swaps with system fonts and there won’t be system replacement font in most computers for font awesome icons.