I also propose that moniker be banned from being self-applied, and is in fact, a smell test: if you encounter a colleague calling themselves a 10x developer, start interviewing immediately. No good will come out of that.
The majority of the team couldn't understand his code. We'd have newly hired senior developers just leave rather than deal with it.
He'd rolled his own code generator for our data model that did everything from model generation to the web controllers.
The result was that while he could pump out work quickly, what would've otherwise been a quick fix for a graduate developer now required a deep understanding of a complex system.
This had the effect of turning what would have otherwise been a team of 1-2x developers into a team of 0.2-0.5x devs with a retention problem.
There was a great deal of thought put into it and he could extend and modify the output really quickly.
The complexity of the system basically made it so that what would otherwise have been a simple task achievable by a graduate required a deep understanding to carry out.
Heh, I've actually done that... twice. Luckily it was a team of 1 and I wouldn't expect anyone else to understand my mess. The code generation was extra ugly since I planned to get rid of it eventually to craft out smaller details. It was great at doing repetitive work in bulk. Not sure if it was actually faster but at least it was less boring doing things that way.
Anytime he "improved" a module, no one else could maintain it as that would entail additional rule-breaking, which was verboten for mortals, so only he could maintain code he touched. Combined with the fact that he didn't add any tests: the net result was he was slowly and surely subverting the codebase into his personal, brittle domain that no one else could change. He was slowing everyone else done, but all management was looking at was his velocity at closing bugs or rolling out new features while creating tech-debt. His boastful personality was just the icing on the cake.
The difference, of course, the framework has extensive documentation.
So that was the missing ingredient.
He made a framework, but wrote no documentation. Writing documentation would have made everyone an n*x developer, for some value of n > 1.