IMHO it is a shame that the informative thread about this bug recently has been locked.
(Who did that and why?)

Description of the problem

When a folder containing thousands of images of high resolution or a single image within such a folder is opened the first time in Gwenview, the program start downloading all these files and generates thumbnail png's in the folder $HOME/.cache/thumbnails.

This may generate a traffic volume of more than 1 GB and take more than 15 minutes depending on the reading speed and data volume. During that time Gwenview may freeze and this bug makes impossible to use the program on such directories.

Imho these thumbnails are not really an intelligent solution of that problem. When the original file of a thumbnail gets deleted, that thumbnail becomes a useless cadaver forever! The thumbnail folder grows without limits and it's advantage becomes questionable.

For exactly this reason Gwenview has the built in possibility to delete everything in the thumbnail folder once the program does terminate!! This however is a silly approach because it also does delete all thumbnails created by other programs and the delay when opening huge folders arises every time...

The better solution is preventing Gwenview to crate thumbnail files. This should be a configuration option. It can however be achieved by setting the attributes of the thumbnails folder to read only.

In this case, Gwenview magically does what it should have done from the beginning: Read the thumbnails embedded in the Exif data of the original photo. This generates considerably less traffic and the delay gets acceptable.

There is one caveat: If you set huge size of the preview images, Gwenview decides not to use the embedded thumbnails and again you get the whole program. I however do not see why one should like to have such huge preview images...

IMHO Gwenview should have in the settings dialogue the option found in other viewers:
"( ) Use embedded Exif thumbnails when available"

Here my workaround:

- chdir to $HOME/.cache
- Eventually delete everything in the thumbnail folder
- Set the thumbnail folder read only: sudo chmod 555 thumbnails

Now the computer tries to generate thumbnails every time a folder is opened and this consumes a lot of CPU when I enter a particular folder with thousands of pdf files. I have disabled thumbnails for pdf files, but is there any other solution? Do you know whether this has been reported as a bug? There is another post on this topic in the forum which is there since 2013. It is strange that this issue is still present.