Opened your project file and took a look. I notice it looks you're using a Google Font for your navigation bar text. You're using a font stack I'm not familiar with, and you've set up some custom CSS to use it as well, so I'm not going to go through that, but I think what you're seeing is the Google Font loading. As is sometimes seen with non-standard fonts, or
@font-face embeds, you're seeing the font load once Google's CDN finishes loading the font. When the font finishes loading it switches from the fallback font to the Google Font.
If this is the case, this isn't anything Foundry has control over. I'm not seeing it here when I load your page or project file, so the font is loading pretty quick for me, but your milage may vary depending on your internet connection.
If you want you can most likely eliminate it altogether by attempting to use a standard font instead of the Google Font.
Also, be aware that you don't need to use an outside font stack. Foundry has font management built in with the Typeface stack. There's some demos on the Tutorials page on how to use it.