focus/start enhancing performance section within pandas/numpy
start of enhancing performance section should begin with tips for speeding up pandas, e.g. no apply, vectorization, pandas methods etc.

Initially I put in:

> This tutorial assumes you have refactored as much as possible in python, for example trying to remove for loops and making use of numpy vectorization, it’s always worth optimising in python first.

I think this should be more strongly worded.

_see [this question](http://stackoverflow.com/q/25676157/1240268), user should not jump to cython!_
