The author here seems to be missing the basic point of the phrase. Others have correctly noted that performance is a feature, but even with that taken for grant

The author here seems to be missing the basic point of the phrase. Others have correctly noted that performance is a feature, but even with that taken for granted, it is not often apparent what the constraints of the full system are until they've been built. You may try and guess, but you're more likely to guess wrong than right. Look at OSes, ISAs, DBs, etc. All of those have been rewritten numerous times not because the originals paid no attention to what performance would be like, but rather because they did not know how to improve performance until they had something they could test.