I have post this question to google groups, please lookup at: https://groups.google.com/forum/#!topic/mongodb-go-driver/NwDvu18faG0

1. the latency increasing while more goroutine are used(from 1 to 8)
2. while I use mongo-cxx-driver to run the test, the latency is stable and low
3. while I use mgo, the latency increases too, but the latency is much lower than mongo-go-driver