It is just slow previewing in a library, with about 1000~2000 photos. This has been a problem since I bought TS-873A three years ago, and never get solved after so many updates.
The CPU usage is low when the photos are loading, the memory has been upgraded to 64GB, where more than 30GB is available for QuMagie. Same problem after a fresh and full reboot of the NAS. Indexing and face recognitions are all finished at the time of test. The system and all the apps are installed on an RAID1 pool with 2x 2TB Samsung 970 EVO SSDs. Original image data lives on a RAID6 pool with 8x 20TB HC560 HDDs. Performance of other apps seems ok.
It really seems a software problem to me. Wonder if QNAP team is aware of the issue.