Hey Greta,
In my experience usually, the number of layers within applications can impact the loading time. Generally I would advise having no more than 10 layers in any one application as each map action (i.e. panning, zooming etc) will send a request to each layer which can really drag performance down.
It is also good to check what each layer contains, and what the requirements for having it are. If a layer is only for visualisation, then having this as a Tile layer or Vector Tile Layer will be more performant than a layer with querying and editing capabilities.
For Hosted Feature Layers it is also worth checking the scale visibility, and geometric complexity, as layers with high volumes of features, or complex geometries can cause performance issues. In these instances I would advise setting the scale visibility to only be visible when zoomed to a fine extent, such as a city.
In this case, it is also worth considering generalising your data, to reduce the number of vertices within it. There are tools within ArcGIS Pro that can do this such as Generalize.
Hope that helps!
David
Esri UK -Technical Support Analyst