InDesign only displays a low-resolution proxy image - and it might be that the proxy image is being generated the retina display, I don't know for sure.
That is, InDesign places a link to the image, and to free up resources, it does not display the image, rather a low-resolution thumbnail. You can control whether it's a low-res preview, or a hi-res preview of the image through the View>Display Preferences, but that's it, it's only showing a preview of the image.
With Illustrator/Sketch/Photoshop, you're looking at the image as it is.
Hmm but why does the image look perfect on the retina-screen and pixelated on the NON-retina-screen? Other way around would make sense.
Also the picture-resolution is fine. It just gets scaled/zoomed in a bad way. In InDesign on 75% (in Illustrator and Sketch ist 100%) it looks perfectly fine! The resolution of the picture is good. When you zoom out/in it gets blurry!
Also Illustrator and Sketch are doing the same thing. Photoshop and Mac Preview both don't have any problem to switch between the two screens.
Here I made some screenshot to illustrate the problem:
The right screenshot is zoomed out by only 1 step from the 100%-view (in Indesign 75%) and becomes very blurry. It is not the resolution of the picture, it is the way it gets scaled.
Since Sketch, Illustrator and Indesign all behave the same it seems like it is a bigger issue with mac or with my setup. Photoshop and Mac Preview though have no problem with zooming on both screens!
Non-Retina Screen at 75% (left), Non-Retina Screen 50% (right):
For comparing I did the same thing with Mac Preview (100% left, 75% right) also on the NON-retina Screen:
On the internal retina screen also Indesign can zoom without blurring the picture completely...
Internal-Retina Screen at 75% (left), Internal-Retina Screen 50% (right):
These options sadly do not change anything. I also played around with the GPU-supported Zoom on and off and all combinations with the live screen drawing. No difference sadly.