[marlin]

EDIT: i realized i answered about btsync while question was about syncthing, sorry.

Regarding detecting file modifications: my friend had issues with his synology diskstation eating 100% cpu with btsync with around 50k files. With less files it was down to 20%. We never debugged it properly but it may be polling issues, but it could also have been out of ram-swapping issues (512MB ram was not enough).

Others have reported issues with many files:

https://forum.bittorrent.com/topic/23802-syncing-600gb-10-mi...

[nodata]

Is Synology ARM? syncthing seems to have a couple of different versions if he wants to try that: https://github.com/calmh/syncthing/releases