For large data sets, it sometimes spends all day in GC while just loading the data into memory (this is actually more of a problem in Ruby than in Python, but it still exists here).

It's also not very efficient about using memory, so it can be easier to get a dataset to fit in RAM in Go vs. Python.

Python also makes it hard enough to make use of multiple cores that it becomes easier to just use Go.

The same goes for making it fast. It can be done in Python, but I tend to be happy with the performance of my Go code out of the box.