With the release of Bootstrap 5.1 a new element was added called Placeholder, this element does just as it says. It shows boxes marking out the site content before the whole page loads. This has not been added as an element or setting in Bootstrap Studio… Yet
You can use the Placeholder classes from the docs here in Bootstrap Studio, that does work, just needs a setting or element in the menu to be implemented.
I would be super excited to hear if this is coming in a future update. Thanks so much for the amazing work put into this program!