I've recently been able to work on performance for our product mainly because our tester has been complaining that it takes too long to run all of the integration tests.

I got to dig in to some hotspots in our code, some hotspots in the test suite code and speed things up significantly. It's incredibly rewarding work. But as the blogpost says, you need to know how things work (and get good measurements) to be able to make a difference. Knowing what tradeoffs to make, such as caching things to trade off memory usage for computation vs reworking algorithms to be more effecient in the first place, is tricky and not something you can necessarily hand over to a junior dev.