There's a delayed load of a minute for customized SharePoint forms in the PowerApps SharePoint webpart.
The customized form displays with a black screen initially for about a minute and then renders. This is happening for two different apps on two different SharePoint sites.
This problem starting happening on 4/30. So there's definitely a performance problem happening with the PowerApps server, service, or infrastructure.
See my earlier 4/30 post on related problem: Apps no longer display using the PowerApps SharePoint web part with app ID: