This is one of my favourite posts. Part of my PhD was based on this post. https://discovery.ucl.ac.uk/id/eprint/10085826/ (Section 4.1). I presented this section of my research in a conference and won best paper award as well.

Another post I love is https://stackoverflow.com/questions/2908822/speed-up-the-loo... where the guy manages to speed up a function which will run for days to milliseconds.