I've known about a student, long ago in the '90s, who got to speed up a batch codebase in some bigcorp, as a summer job. The original did basically everything wrong, up to bubblesort. So student diligently cleaned up the mess and got a speedup from multiple hours to a few minutes.

Then he could not get an interview anywhere. After some backchannel communication it turned out he had to remove that experience from his CV, as all employers assumed he was just lying.