Strange performance characteristics of write ops in shell

db.user.drop();

for (var x = 0; x < 10000; x++) {

    db.user.insert({ x: x });

    if (!db.getMongo().useWriteCommands()) {

        db.runCommand({ getLastError: 1 });

    }

}

db.user.drop();

var bulk = db.user.initializeUnorderedBulkOp();

for (var x = 0; x < 10000; x++) {

    bulk.insert({ x: x });

}

bulk.execute();