Ask HN: How can you practice profiling code?

There are a lot of websites that offer practice problems related to algorithms, data structures, etc., but I don't recall ever seeing any that help you put to use profiling tools. How does one go about getting comfortable with this aspect of development? Are there any websites that have working code examples that you can profile and try to pinpoint what needs refactoring?

I'm particularly interested in JavaScript and Python, though I'm also just curious about how people have learned to become proficient with these tools in general.

I ask because this question has come up in interviews a few times and while I know the very basics of the tools, none of my personal projects ever required heavy optimization. I feel as if this has been a point against me, so I want to get better at it.