If your client is an otherwise idle system with the latest browser, a fast wired connection, and the developers carefully monitor time to render.

In most cases, at least one of those is not true and the developers are using something like CRA so your site has a number of users who see tens of seconds of nothing while their Android phone downloads and runs a 1MB JS bundle. Bonus points if the marketing department has tossed in a tag manager which delays that even longer.