Before user can start the experience, there is an inevitable delay as user’s browser goes away from previous view to the current view.
This delay manifests itself in waiting for first piece of the UI to be painted on the screen. Usually user waits for page to show while looking at the previous page, e.g. search engine results page or another page where they performed an action that led him to the page in question.
On this filmstrip, previous page is shown as white:
The usual cause for such delays are either a bottleneck of a first request for HTML page:
Making it a requirement to start painting quickly is critical, especially as it competes with other technical and design goals of loading large amounts of code and displaying a large number of elements on the page.
Set timing SLAs during product and design discussions: 50-200ms