Slow recovery when scrolling through large numbers of files that are subsequently deleted

[xsdg]

Setup (please complete the following information):

• Distribution: [e.g. Ubuntu] Debian
• Distribution release: [e.g. 20.04] unstable
• Geeqie version [geeqie --version]: git ee78c8c + fixes in Fixes a use-after-free bug in FileCache where a pointer was invalidated by the notify callback system #2340 and Fixes infinite loop + memory exhaustion when reentrant calls to file_cache_get occur #2343

Describe the bug
Discovered this when validating the fix for #2342 . Geeqie no longer crashes or goes into an infinite loop, but it looks like it takes ~1 second per (deleted) image to scroll through ones that are in some kind of queue. And in particular, the UI is not responsive while that is happening. Once it clears the queue, it goes back to being fully responsive.

To reproduce
Same steps as the repro case for #2342 :

1. Create a directory /tmp/gq_test/imgs/ containing the geeqie test images

2. Start geeqie and navigate to that directory

3. Ensure that thumbnailing is enabled (in my case, List view mode with "Show thumbnails" enabled.

4. Run the following command to create a bunch of (symlink) files, wait 1 second, and then delete them all:

   sleep 1; for i in $(seq 1 20); do for file in {a,I,J,R,s}*.*; do ln -s "$file" "xxAA${i}_${file}"; done; done; sleep 2; rm xx*
   

5. While step 4 is running, start scrolling down (towards "xx...") in Geeqie

Expected behavior
Geeqie should drop the deleted files from the list quickly, and should remain responsive.

[virtadpt]

I'm seeing this on Arch Linux, Geeqie v2.6.1 (from geeqie --version) as well.