Cached Image Server Service is Faster as a Layer than a Basemap

[RogerDunnGIS]

This year I published and cached a new image service on our Enterprise ArcGIS Image Server.  The URL is public, so feel free to have a look at it:

https://maps.orem.gov/image/rest/services/OremAerials/AerialCache2408/ImageServer

I have created a map, placed this as the basemap layer, saved the map, and share it with out basemaps groups.  Therefore, users can select this as their basemap.

https://maps.orem.gov/portal/home/item.html?id=20dd0a76ca764ba5850d77c3311d4fe2

However, when this map is used as a basemap, it is slow, taking between 4 and 8 seconds to render.  But when the image service is used as a layer instead of a basemap, it is fast, taking less than 1 second to render.

The image service and map are (should be) in NAD27 Utah Central, but I have my suspicions that the rendering delay is due to reprojection-on-the-fly.  However, the map should be using the coordinate system of the basemap, right?

How can I make this new layer faster as a basemap?

[DavidPike]

From looking at the service it appears to be in Web Mercator as it can be added as an additional basemap layer to esri world basemaps.  Is it perhaps slow because all the other layers are now being projected from NAD27?

[RogerDunnGIS]

Thank you for replying.

The ArcGIS Image Server Service shows that the coordinate system is 32043, NAD 27 Utah Central.  I can't figure out how to get the web map to also have this projection.  I'm pretty sure that web maps inherit the projection from whatever their basemap layer is, right?  Maybe I just need help verifying/forcing the coordinate system of the web map.

[RogerDunnGIS · Accepted solution]

I was able to resolve this issue.

Previously, I had added my image service to a new web map as a layer, moved it to the basemap, removed the basemap that was there previously, and saved the map.  Of course, I shared it with appropriate groups.  This time, I went into the web map's basemap properties and added a new layer to it via URL.  The basemap now works quickly, as intended.

One of the ways I figured out what was going on, was I created a blank map with the old basemap, saved it, and took a look at its JSON in C:\arcgisportal\content\items.  I then did the same thing with the new basemap, saved it with a different name, and looked at its JSON.  Both blank maps had the same coordinate system (32043), but in the old case, the image service was being treated as an ArcGISImageServiceLayer, and in the new case it was being treated as an ArcGISTiledImageServiceLayer, which I guess made all the difference.  My guess is that ArcGISImageServiceLayers are treated as if they didn't have a cache (?) while ArcGISTiledImageServiceLayers are?