Yes it is, but if you're doing the first 90% properly you have a much better shot at mitigating the difficulty of the last 10%.
I think there's some vague point in any project where it goes from being 'easy' to 'hard' to add new stuff. Basically the only factor that matters for productivity is how long you can delay that point. If you just do the first 90% as quickly and cheaply as possible, you're just resigning yourself to hitting that point as early as possible.
I think this is best explained without exaggeration by the famous Design-Stamina Hypothesis[1], which states the notion that time spent on Design is something which you can trade away to improve development speed, is reliably false in the long-term (even if it seems to be working in the near term.)
The graphic also suggests that there is an inflection point, as you suggest, before where time spent on design really is just slowing you down in the beginning of your project, but also that the costs of waiting too long to switch modes (from doing no design, to doing good design) after you have crossed that line, are substantial and compounding; the longer you wait, the more your lack of good design costs.
And of course, not pictured, is "bad design" which can be even worse than no design. Trying to find that inflection point and put it on your development schedule in advance is also a bit like trying to catch a falling knife (not likely to succeed.)
I think there's some vague point in any project where it goes from being 'easy' to 'hard' to add new stuff. Basically the only factor that matters for productivity is how long you can delay that point. If you just do the first 90% as quickly and cheaply as possible, you're just resigning yourself to hitting that point as early as possible.