I've been using NumPy for nearly five years, but I'm still learning performance tricks. The reason is that I currently need to deal with very large arrays (hundreds of millions of elements) and the performance of my code started to be disappointing. Then, through extensive line-by-line profiling, I discovered some subtleties that explain why some seemingly harmless lines of code can actually lead to major bottlenecks. Very often, a small trick allows to significantly improve the performance. Here is what I've learnt. These tips are intended to regular Numpy users rather than pure beginners.