mongod outputs all (slow?) queries, in full, to the console (even with profiling disabled). This is an issue when using $in with a very large set of values/ids. As detailed in the post below, specifying some 130 000 ids for retrieval causes the ENTIRE QUERY to be output to the console, meaning some 50 000 lines which takes considerable time.

http://groups.google.com/group/mongodb-user/msg/4045e74f8756b4ef