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.