@protro I'm not familiar with rockfrontend. As you can see in a browser console your final code loads several files (uikit.min.js, uikit-icons.min.js, main.js, main.css) before loading your oswald.css file with the @font-face rules. The browser reads your oswald.css file and then sends a request to download webfonts.
So two things delay the webfont download. The order of your js and css files and also the chaining via the font-face rule inside a css file.
Most of the time it helps just to change the order and to set the css file at the first place.
And as I said before the fastest way would be to inline those @font-face rules.
If inlining does not help also, you could also give the browser a hint to preload webfonts like this:
<link rel="preload" href="/site/templates/styles/whereever_your_fonts_are/webfont.woff2" as="font" type="font/woff2">
<!--
or for crossorigin
<link rel="preload" href="https://www.domain.com/webfont.woff2" as="font" type="font/woff2" crossorigin>
-->