Just try it, title says it all, disabling Real-Time-Protection solves it. It does not even make only syncthing unusable, but the whole mac slows down until it crashes.
What also solves the problem is uninstalling Avira completely, which is what I did, it has been annoying me for quite some time now.
Not really a bug report (not even sure whether this is Syncthings “beer” or Aviras), I just wanted to post this here, and save someones time, in case he has the same problem.
Cheers