Are you using FontAwesome for anything? I think that's the CDN they use... and if so, it means that you have a slow connection to them.
You can try hosting your FA locally instead of relying on the CDN.
if you are running any of the UI.X family styles, should be an option somewhere to enable/disable the CDN. There is also the iconator - ACP -> Appearance -> UI.X -> Options -> UI.X Iconator and select use local - that should load them locally. If you have the gallery or other add-ons that use fontawesome they also may be loading the call to the CDN. You need to check to see if they have an option to use local or go in and comment out the template and then put it in the template to load the font locally (similar to how you do Google Fonts.
There is also an add-on that will do this. https://xenforo.com/community/threads/font-awesome-install.93291/#post-901688