Bug 460126

SUMMARY
When browsing a remote fs, Dolphin is extremely wasteful about thumbnail generation.
Dolphin does not seem to cache any previews and re-generates previews for a file on every encounter. This is bad for the following reasons:
- wear and tear on hard disks
- network usage
- slows down browsing, even a moderate amount of files, nothing out of the ordinary, can grind Dolphin to halt; one has to wait seconds on every navigation before being able to perform the next navigation

STEPS TO REPRODUCE
1. Ensure you have a remote fs
2. Open iftop
3. Open Dolphin
4. Ensure Dolphin detects your fs as a remote fs (check whether it's listed under "Remote" in the sidebar)
5. Navigate to any directory that contains files which Dolphin generates previews for, observe traffic spike in iftop session, observe thumbnail generation
6. Navigate to a different directory
7. Navigate to the initial directory and observe traffic spike in iftop session, observe thumbnail regeneration

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux (x86_64)/5.25.5
KDE Plasma Version: 5.25.5-1
KDE Frameworks Version: 5.98.0-1
Qt Version: 5.15.6+kde+r177-1

ADDITIONAL INFORMATION
The remote fs that I used for testing is a SMB share mounted via CIFS. Dolphin correctly detects it as a remote fs, it is listed under "Remote".