I've had junior engineers outperforming senior engineers, even counting the hours I had to spend correcting and guiding.

Some engineers are just slow. I don't think it's much correlated with experience. If your goal was never to be a super productive programmer, I don't think you'll ever become one.

This might also be why there is this spectre of agism in the industry. Everyone knows there's greybeards out there that would eat your lunch and get paid the big bucks, but at the same time older programmers have hard times interviewing. At that age most of the really great programmers have probably already been fished out of the pond.

I think its important to understand there are also relatively high floors to getting certain things done well. If the engineer is too junior or simply not good enough, its not that you will get there slower, its that you may not get there at all. Or you get there slowly and with a very buggy or unmaintainable mess. Some engineers can even go backwards in that sense, attempting to optimize the wrong solution and spending months on something that in honesty can't actually be completed.

Cue the link to Mythical Man Month:

central theme is that "adding manpower to a late software project makes it later."

https://en.wikipedia.org/wiki/The_Mythical_Man-Month