> if AI can reduce the time from 0 to N by 10x, the time from N to full product is also significantly reduced

Only if you assume that progress is entirely one-dimensional and there are never any trade-offs. We know both those assumptions are incorrect.

An extreme example: leaping out of my office window gets me down to the ground much faster than taking the stairs. Because of recovery time, it does not at all speed my journey home from work.