3 shards, in 3 physical servers, 3 replica sets. Each replica set has 2 members + one arbiter.

I have 50 threads inserting a document 1000 times each, 50K docs in total.
after 30K writes, the process hangs, with currentop showing getlasterror(s) queries running for several thousand seconds. Mongostat show no activity, no locks, no inserts. No errors show in the logs. All inserts are performed with REPLICAS_SAFE
I attach output of db.stats, db.serverStatus and db.currentop.