Import - folder list in target panel opens slowly with files stored on a NAS

I have about 90.000 photos stored on a Synology NAS, connected to my Mac Powerbook Pro (2022) via Ethernet. My LrC Catalog (about 2 GB) is stored on the Mac. LrC is kept in the most updated version. Since a while now I'm having the following problem:

I have physically organized my photos on the NAS in folders by year with more subfolders inside each year's folders. When importing photos into Lightroom, trying to open the NAS folder list in the target pane on the righthand side takes ages. Simply clicking on one of the annual folders to unfold it and see the subfolders in it may take a couple of minutes with the wheel spinning. And frequently, after a couple of minutes the connection to the NAS is suddenly broken during this process. While LrC shows this behavior, I can easily and quickly browse through these folders in the Finder - until the conncetion if broken.

I thought, syncing the main folder on the NAS (which contains all annual folders as subfolders) could cure this. However, when trying to sync this folder, the connection to the NAS is also suddenly terminated during the process.