when same query and index run on 2.6.4 it is much faster then on 2.6.5 , looking at explain show second scan to not use the indexes.

the attached js file has both query and explain results for both versions, can see millis difference . 150015 on 2.6.5 compared to 0 on 2.6.4 server.

follow is [ObjectId()]

query use $in and $nin .

the affect is noticed when users is more then 110000 documents on a server with 1GB RAM.