Python works well and is stable. If the performance problems were localized to a few components we would have used C, Cython or Pypy. The issues were quite fundamental though. Things like serialization, object creation, ranking and aggregation are very hard to speed up using Python. We worked around it with very extensive caching strategies, but it was just not as fast as we wanted it to be. Performance is a major selling point for our customers so we felt we needed to fix this issue. Go has been a major win and our team is very happy with it.