Development is a funny thing. A lot of it depends on the potential of the car. A lot of it also depends on the timing, and how big the flaw on the car you are correcting through development is. If you have a lot of small easily correctable flaws, as the 2019 RP had, then progress will be more noticeable.
On the other hand, if you're trying to squeeze water from a stone, progress won't be as easy to notice.
Most of the pace improvements still come from the power unit. The aero is so complex, that even guided by CFD and good aero engineers, there is still a fair bit of trial and error involved.
The height of cultivation is really nothing special. It is merely simplicity; the ability to express the utmost with the minimum. Mr.Lee