Qnap has responded to the problem (why it takes so long for thumbnails to appear and then disappear/reloading when scrolling up or down. I will post when I hear there is a solution. I have informed Qnap that this problem is present even when I have 500 photos in an album (essentially links to the large photo gallery of 24000 photos).
Qnap response:
“The issue is caused by a bug affecting QuMagie in certain circumstances, not your NAS specifically. The issue affects the handling of large photo libraries and jump navigation.
The developers are working on this bug but there is no ETA for when it can be fixed.
Currently there is no direct workaround to completely avoid this behavior but as a temporary workaround the developers suggest:
>Use Filter → Timeline to limit the browsing range (for example, selecting a specific year).
>Use Album / Folder filtering to reduce the number of photos displayed at one time.
>Avoid frequent large timeline jumps when browsing very large photo libraries.
These methods can help reduce the amount of data loaded during browsing and improve responsiveness.”