Yeah, I think type instability is definitely the biggest culprit when someone's ported over some Python or Matlab code for the first time and isn't getting as much of a speedup as expected. It isn't hard to fix, you just have to know about it (and how to check for it with `@code_warntype`).

That and perhaps the excess unnecessary allocations from array indexing on the right hand side of an assignment, if you don't know about `view`s.