That all is quite relative. From my point of view, nothing can be done about Syncthing. For me, the problem clearly lies with the DS.
You have to consider that a DS120j has DDR3 512 MB RAM, which is not much when you also consider that DSM 7 also has to deal with it and consumes more resources than DSM 6, at least that is my experience.
Furthermore, 6 folders do not read very much, the question is how many elements are contained in them, i.e. folders and files each. In my experience, each element takes up about 0.8-1 kB of RAM, because Syncthing has to manage all these elements continuously.
If we take this aspects into account, the solution could be to remove elements or to install more RAM, if this is possible with this type of DS, or to use a DS with more resources. Unfortunately, the fact is that the DS120j is not strong.