Potential causes:

• Lazyloading of sprites
• Firebase authentication function (this loads very slowly in the Inspector)
• Browser and/or browser extensions (ad blockers, script blockers, etc)

This shows up in preview URLs and the public site, but not on the localhost development instance. Not all users see this, either. It loads quickly on my phone and iPad but slowly on my laptop in Chrome. Chrome Incognito Mode in the same browser installation speeds loading up considerably so there may be something to do with browser settings/extensions.