New features are visible, performance improvements are not. People, even here, would probably complain about the lack of new features if they decided to “take a break” for a month to focus on performance.
Not to mention that they couldn’t do that anyway. Not everyone can contribute to performance (for example, I don’t think there’s much performance improvement in the sound department), so there are people that could only do two things: Prepare new assets (for game ideas that are not fully developed/thought through due to a lack of time, which might be a dangerous timewaste) and do nothing. There’s only so many asset one can prepare in advance.
Personally, I think as soon as the game has all the core content required, they may shift their focus from new features to performance. But then again, I’m being really pessimistic and say that cities, such as we saw in the Kickstarter, will likely not make an appearance in the final game.