this might sound silly, but yesterday and today I had to write an image to a sd card with dd. Each time Syncthing started immediately to scan my shared directories. I guess that this slows down dd.
Is it possible that dd triggers syncthing to start scanning?
If its separate storage devices there shouldn’t be a slowdown. If you are using inotify, then it depends on what the write means. If you are dd’ing to a file, and using inotify, then yes, I guess its expected that inotify would pick up writes to a file.
If you are dd’ing to a block device or have no inotify, then its just a coincidence as we rescan once a minute by default.