Why is the REST service for soil map so slow ?

[MarcWouters]

In our C++/Qt runtime application, we want to use a soil survey map as basemap.

 ArcGISTiledLayer* tiledLayer = new ArcGISTiledLayer(QUrl("https://server.arcgisonline.com/ArcGIS/rest/services/Specialty/Soil_Survey_Map/MapServer"), this);
 _map->setBasemap(new Basemap(tiledLayer, this));

This used to work fine.

Since a few weeks we started to re-write our application using Runtime SDK 100.0.0 (instead of 10.2.6), and I hadn't used this basemap for a while.

Now I wanted to use it again, but the map is incredibly slow, or even not loading at all.  (see the picture in ArcMap)

What's happened ?