Lines or amount of code written is a terrible, and easily gamed metric.

Some of the very best engineers write the fewest lines of code, often making fewer yet better abstractions that fulfill more business use cases more simply. They might also help others write less code.

Some of the most prolific coders are great. And some are terrible. The challenge is discerning the two. Coaching a prolific yet bad coder to slow down can be as challenging as coaching up a slower / not yet confident coder. And the slower coder will do vastly less damage in the interim.