You can see this running the unit tests locally. The test suite hangs for some time before completing. This is also causing confusion in the community, with users assuming PyMongo 3 is "slow" for certain styles of benchmarks. For example:

http://stackoverflow.com/questions/30334438/pymongo-3-0-2-slower-than-2-8-1/30338779#30338779

The problem appears to be in periodic_executor.py. When the close() method is called self._stop is set to True but there is no call to wake(). This means the thread blocks for up to 10 seconds before waking and terminating.

https://github.com/mongodb/mongo-python-driver/blob/master/pymongo/periodic_executor.py#L75