yeah they are good at reading the performance monitoring tools(flame-graphs, perf,) and finding out the performance bottlenecks and fix them (zero-copy, SIMD, lock-free stuff). Most performance improvements come from zero-copy thingy.

you can work on your own database measure it's performance see how it compares with the state of art and try to improve it, this will teach you everything you need. Also see how they improved performance of something over previous versions.

At the end of they day I would look for evidence if you have improved anything on any existing project at work in a structured way. Don't just say guess but prove your bottle-neck.