A couple of things:
If you go to that endpoint, you will see the following key - Single Fused Map Cache: true
This is basically synonymous with a "cached tiled map service", so you should use the ArcGISTiledLayer class for this.
I'm actually not really sure how it is working with the map image layer. Maybe that is the hang up?
Another thought could be the 5.8 networking bug (not sure what version you are on) - https://community.esri.com/community/developers/native-app-developers/arcgis-runtime-sdk-for-qt/blog/2017/06/15/important-notice-about-qt-framework-58-and-arcgis-online-tiled-layers
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Yea, it would be interesting to watch the traffic in Fiddler or something like that. My guess is the Server takes a bit of time to reproject all of that local raster data before it sends the images back. It is certainly not a trivial change, so a performance hit would likely be expected...
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