[flukus]

> In fact, what performance oriented devs need to be taught about these days is how to not use hash tables, or, more generally, dictionaries. They are a super-easy data structure to reach for, but often it is possible to arrange your data such that their use is unnecessary.

A lot of devs are completely unaware of what's happening at the layer of abstraction below them and this is one of the ways that comes out. The number of elements it takes before hash tables are faster than iterating through an array can be surprisingly large and yet it's one of the first "optimizations" that get made.

Some other related problems are not knowing how expensive network calls are, not knowing what the ORM is doing, caching in memory even though the database already is and more. They just don't think about performance issues in code they don't write.