I think there's also an issue that a software engineer's job changes as they're more tenured at an organization. When you're newer, no one expects anything of you other than to come up to speed and build new stuff. If you have a project that doesn't require deep knowledge of the existing systems, push it to the newer guy.

Only once you have deep knowledge of the systems you work with does it really make sense to give you work like a complex redesign of a group of components in an online system. And you then have a responsibility to help new team members learn. You do more code reviews, and more consulting on projects for which you are not a primary contributor. And you get ugly projects that are related to your area of demonstrated "expertise". Is that "slowing down"? Or are you just doing different kinds of stuff? And is it because the engineer stagnated, or that the organization created a morass around them?