Enormous network payloads
This is a technical term for large file sizes loading on your website.
Loading time can be reduced by not having large files loading at the start of your website (keeping them “below the fold”), or by minimizing the size using an app.
You may not notice the slow down from large assets because your browser will cache them so they load faster the next time.
Try viewing your site in an incognito window so you can experience it as a first time visitor might.