Several links to symptoms of the issue. At the end of the day, this looks like a problem in 2.0.x but not 1.4.x – which is a huge problem since I just switched all my code to 2.0.x. This bug has severe performance issues on queries, reducing performance anywhere from 25x - 100x.

http://stackoverflow.com/questions/27830277/odd-mongodb-node-explain-results

http://stackoverflow.com/questions/27831936/limit-1-1ms-limit-1-150ms-mongo-melt-down

http://stackoverflow.com/questions/27844128/mongodb-native-node-driver-explain-is-broken