Nowadays it is common to make a distinction between a thumbnail and a preview image when raw formats are concerned. All raw formats that matter contain an embedded image of some sort, some contain several, e.g. some Nikon raw formats contain 3 embedded JPEGs: one is a full-sized preview, one is a half-sized preview, and one is a small thumbnail. Extracting any of the embedded images is fast; generating a thumbnail from the raw data is slow.
The link I pasted in my previous post suggests that there are at least two ways for Nautilus to get its thumbnails, and both of them suck. One method’s program is “terribly slow” and dead since 2005, the other “fails with 256px thumbnails” whatever that means. No info is given as to whether either of them use any of the embedded images or generate an image using raw data for the thumbnail.
If you want to see raw thumbnails, consider using something modern that isn’t born to lose: darktable, digiKam, Dolphin, Gwenview, Geeqie.