Using the test case from SERVER-25865, in 3.3.9 and later, for the duration of the unusally slow $group operation, marked by the interval from A to B below, no FTDC samples are captured:

This did not occur in 3.3.8:

Since the $group operation in the test case from SERVER-25865 is only slow on Windows, we can't tell from this test case whether this problem only affects Windows or also affects Linux.

Also note that while the FTDC stall only occurs in 3.3.9 and later, the slow $group operation on Windows from SERVER-25865 occurs all the way back to 3.2.

FTDC data for 3.3.8 and 3.3.9 is attached.