Using Nautilus (File browsing) with this extra tricks you can get your CR2 files thumbnail to show up. However, this is very slow, sometimes taking ages to load.
If uploading to Google Drive and using the web interface, the thumbnails display relatively quick, if not instantly.
What is GDrive doing that is far superior compares to Nautilus?
Gdrive has massive amounts of developers, storage, and compute power behind it. Nautalis does not.
They are likely generating jpg thumbnails for every raw you upload at various resolutions. And they have a couple of fast computers to do it with. 
I had the same problem and deinstalled it for that reason. Other software, e.g. darktable, is really fast in reading the embedded thumbnails, but from the speed I guess that nautilus is doing a raw development for each file. You should file a bug report.
Read the README: GitHub - angryziber/gnome-raw-thumbnailer: Fast and rotating raw photo thumbnailer for Gnome/Nautilus
Gwenview loads CR2’s very fast (about as fast as the corresponding JPG’s). So there exist an efficient FOSS lib somewhere for this…
Each raw format (practically all, but not all) contains an embedded thumbnail.
Practically every simple image viewer simply displays it. That is as fast as it gets.
Right… Got mislead by the fact that the “thumbnail” is actually a full-size image (quality 80, chroma halved, it seems).
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.