If a programmer is not able to write efficient code, they may not even know that their code is inefficient.
I regularly see programmers that come from a front end or "full stack" background try to solve data engineering problems, and end up with solutions that are 10x, 100x or sometimes 1000x slower than what they could be expected to.
It's not rare for me to catch Pull Requests with horrible performance holes. Like 'SELECT * FROM...' executed inside a loop where a single 'SELECT id FROM...' outside the loop could solve the problem.
I regularly see programmers that come from a front end or "full stack" background try to solve data engineering problems, and end up with solutions that are 10x, 100x or sometimes 1000x slower than what they could be expected to.