I have been affected by this too.
And this is relevant.
I made a directory with all the images [~500px width] required for the thumbnails and a .directory file in multiple subdirs of a directory on a removable HDD.
When I first open it after a restart, it tends to take a few seconds for the directory to load.
It would be good to have the directory contents load first and let the thumbnails take their own merry time.
Also, this implementation would be augmented by the automatic parallelisation/serialisation functionality in KIO.