In my case: I have EKOS installed on RPI4 (4Gb) and I uses VNC to remote from my MacBook Air 11". I set VNC the resolution is 1366 * 768 which matches the monitor display. My imaging camera is ATik 490ex which the image size is 3379*2903. You can see the image at a much higher resolution than my monitor. 

At my imaging session, I usually has to zoom in and out to check a few things:

At low resolution, I wanted to see the overall image like the object position, orientations are good.
At high resolution, I'd like to check for alignment (i.e. any star trailing) or may be focus shift or look for other artifacts

For my case, loading the full image into the memory is in fact a waste of resources - my monitor cannot display all the pixels.
If I want to check focus shifts or alignment problems I only need to look at part of the image to find out.

I'm not sure a magnifying glass approach would be worth to look at? Like the FITS viewer would initially show the preview at low resolution, which can be pre-generated. Display this low-resolution at the FITS viewer. Then on mouse-over (or may be a mouseclick is better) to load that part of the image from disk with an area say 50x50 and show that part as 1:1 (or may be magnified 2:1) in a "magnified" image box?

Stephen

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