When your code is too slow and you checked your code for bottlenecks.
http://www.marinamele.com/7-tips-to-time-python-scripts-and-control-memory-and-cpu-usage
https://www.amazon.com/High-Performance-Python-Performant-Programming/dp/1449361595/ref=sr_1_1?ie=UTF8&qid=1416157131&sr=8-1&keywords=high+performance+python&pebp=1416157123265
https://docs.python.org/3.5/library/resource.html –> Resource management in python
https://www.tensorflow.org/tfrc/ –> tensorflow computing servers for researchers (free)