I have a feeling that implementation speed is an instance of Goodhart's Law (https://en.wikipedia.org/wiki/Goodhart%27s_law). If you keep everything constant (code quality, amount of tests, documentation, etc) then a faster engineer is obviously better. However, if you start using speed as a criteria to judge engineers, then the easiest way to increase speed is to sacrifice things which make code maintainable and modular.

Finding metrics which work well even when people try to game them is incredibly difficult (if not impossible).