Hate to side track, but how did you insert your own image and colour during the loading screen? (first screen that appears when you click your link)
Theo,
I branched your question so you did not hijack that thread. In WAB you have the attribute tab on the left side of the builder app. In that tab you will find a Loading Screen > link that you click on to set the color and loading image for your app.
(originally question was posted here)
https://community.esri.com/thread/187032-performance-of-web-app-builder-app
