> db.$cmd.sys.inprog.findOne()

{"opid" : 15209181 , "active" : 1 , "secs_running" : 0 , "op" : "update" , "ns" : "en_gb_PROD.visits" , "query" : "" , "inLock" : 1}

in that output, it would be nice to have the update query so we can see why one is slow.