@christoph_s I tested just now on Windows 7, not from the Explorer (I have self made builds, not installed ones and so don’t have context menu entries), using the -R flag (which is used in this context menu entry), and it opened in 5s a folder with 300 raw files where thumbnails had to be built again. I saw the main window in 5 seconds, the embedded thumbnails were all displayed few seconds later and then it started to process the thumbnails. The thumbnails work was a background task. RT cache on SSD too here.
I tested again by setting a file name instead of a folder name in the command line, same result for the file browser, and the reading of the image started once the main window was opened, i.e. 5s after starting the app.
I copied the big folder (1560 raw and jpeg) on an external drive connected with USB3.
Cache deleted, I made “open with rawtherapee” on a few files.
Waiting time until able to modify a slider in editor about 9s.
I think that there are remaining places in the code that blocks the GUI by doing heavy processing instead of being done asynchronously. Maybe there’s only one remaining and due to race conditions, it blocks on one system but not others. Just my 2 cents, but someone should have a look. I’ll open an issue to review the code on this when back home.