I have seen a very clean example of this once -- we hired two people at about the same time, let's call them A and B. Both had about the same years of experienc

I have seen a very clean example of this once -- we hired two people at about the same time, let's call them A and B. Both had about the same years of experience and worked on similar technologies in the past. Neither was familiar with our codebase. Both got the same onboarding process (1-on-1 with experienced developers few times a week, simple tasks). Then both could choose tasks to work on from the same pool. Two month later, A is done multiple simple tasks and in the middle of refactoring a tricky module to add much needed functionality. B is still working on their second simple task. I would not be able to quantify difference between them, but it could easily be more than 10x. (We later tried to see if B would do better at other projects -- a different programming language, a project that needed a small program from scratch instead of modifying our huge main codebase, etc... None of that did help.)