Julia is quick, but poor coding can slow it down markedly. If you are developing enterprise software and/or distributing on GitHub, you need to pay more attention to some of the features and nuances of the language.
The online manual has a long section on performance tips, including a useful style guide and a section of frequently asked questions, which will prove to be illuminating. I've cherry-picked a few that I found of interest.
First, a few points about variables, arrays, and types: