I got an cursor from the simplest query possible

(b.some_collection.find()),

and if the collection is big, the iteration on the cursor will pause after the first 101 docs are iterated (the 1st batch plus 1 more doc). The mongod process is doing a lot I/O reading. It looks like it's loading the whole data in. 

That will make my code freeze for a long time for large collections. The same thing happens no matter I do the query via pymongo or in the mongo shell.