The file in quaestion was not large at least it was not too large. It was 250 mb( the largest file in that particular share), and yes setting weakhash to 101 finally fixed the file that was not syncing for a week.
I saw Syncthing using something like %35 of my physical memory (12GB) around that time, I checked it because the UI was constantly timing out. So it was syncing that was eating my my avail memory. It never gets that high generally.
As far as I can tell things got smoother after the new setting.
Please see this one Poor performance on big file and the post I replied to.