The majority of work these days are web apps, and the bottlenecks are usually database query and IO issues. Many can be solved with an appropriate index. If you have a slow Django app, I would guess than 9 out of 10 times it is someone who is letting the ORM produce far too many queries in loops. Hashmaps are never something I have needed to look into personally, but I guess it depends on exactly what you are coding.

There's quite a bit of data wrangling happening both in the backend and in the frontend.

Rereading your comment, do you mean just using a hashmap / dictionary, or do you mean writing your own hashing functions? The first is invaluable, but I have never had the need for the second (pretty sure you could define your own hashing functions in Perl, but I never found the need).