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I am trying out shooting RAW+JPEG to see if that workflow is what I want. I am surprised to see that Lightroom still shows the "Loading" text when I zoom in on an unedited JPEG file. I expected that Lightroom would not need to build a preview of a JPEG file, as that already is a preview by itself.
Why is Lightroom loading when I zoom in onto an undedited JPEG file?
Don't forget that LR is a non-destructive editor. This means that for all sources, whether RAW or JPEG, LR does not alter the original in any way. It does this partially by building a database of the image itself, so that it renders changes on screen, mimicking how they would look in the final export.
LR builds several previews, stored in the preview database. It does 'normal' previews when you import the image, but if you zoom in, the render may not have enough data to show the zoom properly, thus LR may have to do another render with more detail. These are much larger in file size and take longer to build. One option is to have LR build these on import, which will result in longer import times, but no need to wait for LR to render an higher quality preview.
Finally, I believe this is the real answer:
With RAW, there is only a small, low quality JPEG preview within the RAW file available, so LR has to create better versions for its use. As you observe, for JPEG originals, it would be possible to simply use the JPEG file itself, rather than render a preview. But that would mean that LR must have two different behaviors and methods: one for RAW, and a different one for JPEG. Very inefficient.
Using the JPEG as the source for a preview also means that you must have access to that JPEG at all times. Certain functions in LR work without access to any images. Since LR is using its database, the original can be offline, or elsewhere, and Library functions are still possible...if you have the needed previews. However, Develop module features are not available without the original.
A lot of image view/edit softwares produce thumbnails of highres jpegs as it still takes quite some time to load a full size jpeg (note windows 7's hidden thumb.db files for example). 10Mpixel and above will also be quite slow to access from memory even on modern computers.
Thus, internally softwares will rescale the images to the view as to not perform gamma, contrast, etc. corrections on the full resolution if you are only showing part of the image and another scale.
All this is to ensure a more fluent experience for the users (like its better to show a blown up lowres version while loading the highres version than to have a black screen in between), but the step-in and -out from these scale-spaces will then seem like a strange step to the user.
Unless you ask Adobe, you will not get a definite answer but I can tell you why this is beneficial. I spend nine years working on a real-time image processing application and the way to achieve high-performance is to use something similar, we called it a Cache and Lightroom calls it a Preview.
The basic principle is simple, you speed up loading by having images at various resolutions and then loading the smallest one needed. Even though a typical JPEG can load quite fast, when you have a grid view, you can show dozens or more. Having smaller files, lets you load considerably more. Then, when a larger is needed because of a user zooming in, you load the bigger one. Some JPEGs can be huge and you can save minutes of waiting with this approach.
There are even further ways to optimize this and one is to make sure the images are saved in a format which is faster to decode. I have no idea if Lightroom does that but it can give a considerable speedup. The more you pay attention to things like alignment, block size, etc, the more you can optimize.
