If the highways are crowded, and they build more lanes, the highways get more crowded.
If development is slow, and you add resources, development gets slower.
Adding people to a project increases the capacity for activity. Activity doesn’t translate to outcomes.
In these cases, you’re adding capacity to the system for cars, or for work, but those aren’t what makes the system run faster. Instead, adding capacity for traffic or for activity leads the system to change in ways that generate more traffic or activity. Which gets in the way of flow.
(lots more examples in this article)
What you want instead is to make flow easier. Add trains, intersperse commerce and residential. Add continuous delivery, add support structures to make progress easier. Don’t add more capacity for work! Doing work isn’t the point! Make the path shorter, instead.