Major - P3 looks like inserts are incrementing along with updates? Are you counting PK index updates perhaps?
mongos> db.serverStatus() ... "opcounters" : { "insert" : 0, "query" : 0, "update" : 0, "delete" : 0, "getmore" : 0, "command" : 2334 }, ... mongos> db.foo.insert({"_id":1}) mongos> db.foo.update({"_id":1}, {$set:{"foo":1}}) mongos> for (i=0; i<1000; i++){ db.foo.update({"_id":1}, {$set:{"foo":1}}) } mongos> db.serverStatus() ... "opcounters" : { "insert" : 1002, "query" : 0, "update" : 1001, "delete" : 0, "getmore" : 0, "command" : 2354 },