A group within my organization is developing an app using Web AppBuilder. I've noticed that the map appears almost immediately, but then it takes a long time for the header/navigation elements to render. They are using the Tab theme, but I noticed the same behavior when testing different themes. I have a video below which shows what I'm talking about.
When I looked at network tab on my browser's developer tools, it appears that requests to all the various layers in the map must be completed before the header/navigation elements render. The app in question does have many layers, and changing that is not an option.
Does anyone have any insights as to whether I am experiencing the expected behavior, or if something is amiss?
Unfortunately, no. I had a support call with Esri. There was no solution. And I guess that now Web AppBuilder is on the road to deprecation, there won't be any improvements for this template.