Please share your kinfo result. How slow is the drive you are testing with ? Is it a network filesystem or an HDD ?
I am guessing this is worse with bigger images.
That might show that the issue is just reading the file. The information panel will read the file to extract metadata from images.
The image reading shouldn’t ever block the UI due to the image being generated out of process, by a separate kioworker thumbnail process.
So I suspect that’s more due to the underlying fs, and a simple stat or “read” metadata to be very slow. This should be out-of-process too though.
Audio files, mp4 could/should have the same issue.
Or maybe your are using a ubuntu based distro and hitting Kioslave 5 closed unexpectedly - #20 by inv-er5kar