Mongostat shows 2 updates on Primaries when insert data
I haven't confirmed by looking at code but I'd bet they are updates to the local database regarding your secondaries.  The primary keeps track of the current status of the secondaries in the set (including where they have synced to), so that would be an in-place update to the local.slaves collection.  Similarly, there is local.replset.minvalid which will certainly be changing as you do inserts.

Basically, I wouldn't worry about it, it's not going to be a major factor in your performance (by the way, if you are using 2.6+ then you should read this Q&amp;A on performance of that type of loop in the shell).