This is a commonly asked question on the forums. In the future, please be sure to search for existing threads before creating a new one. 
This issue is called FOUT (Flash of Unstyled Text) and you can read more on this here: https://webdesign.tutsplus.com/articles/quick-tip-avoid-fout-by-adding-a-web-font-preloader–webdesign-8287
You can fix this by adding a bit of custom code to your site — Dave explains this more here: Flash of unstyled text - fallback fonts loads initially then switches to google font
Alternatively, you can try downloading the Google font, then uploading it to your site as a custom font.
I hope this helps!