Super strange, let’s debug and figure out what’s going on.
Try disabling Photon in “Jetpack > Settings”.
I ran a test on your site, and it looks like Photon is returning a 504 bad gateway on one of your images, causing it to take over 10 seconds to load that single image.
Another thing I noticed, this file: https://cdn.rawgit.com/twbs/bootstrap/v4-dev/dist/js/bootstrap.js
It’s loading way too high in your source, above where jQuery is loaded, which won’t work.
How are you adding the above file? GP Hooks? If so, load it in the “wp_footer” hook.
If none of the above helps, let’s go through the following process:
1. Do you have a caching plugin? If so, can you try clearing the cache?
2. What happens if you deactivate all non-GP plugins? Still the same issue?
3. What if you deactivate GP Premium? Is it the issue gone?
4. If so, re-activate GP Premium and go to “Appearance > GeneratePress”. Deactivate the add-ons you have activated on by one. Does the site speed up once after deactivating a specific add-on?