Reply To: Expanded Column Background comes late

The reason for that is simple. The expanded background is calculated with JS, which obviously is loaded at the very last of the site, right before the body tag closes.
The actual background is set in CSS, which is loaded inside the head, so it will always render first.

It’s not a browser issue, it’s just a technical thing.
So how long is the delay between the initial BG and the expansion? This depends on your hosting, your page optimization, the actual page size, HTML DOM, etc.
The only workaround I have in mind, this feature should only be used for areas which are not visible on pageload (so not within the first 100vh).
You could also consider other solutions, like using page transitions, a page intro, etc.
If you really need this on the starting screen on a particual page, I would recommend you to solve this with custom CSS.

Other opinions and technical point of views are always appreciated.