The React team did say, many times, that React 16 was not intended to be a "make everything faster" release. The "React Fiber" internals rewrite was intended to enable making re-rendering asynchronous in the future, but for now, the overall rendering process is still done synchronously to maintain compatibility with 15 behavior-wise. It's possible that the changes may make things smoother for your app now, but this was not a pure-perf-oriented set of changes. (The future asynchronous rendering is also not strictly speed-related either, but rather opens up a lot of possibilities for how React prioritizes updates.)
The initial improvement that Fiber gives is about splitting up the rendering process into bite-size chunks so that the determination of what does need to change doesn't block the main thread. The rewrite of the internals also made the codebase more maintainable, and gave them a chance to implement often-requested features like returning strings or arrays from render(), as well as implementing error boundaries.
You know very well that community doesn't hear nuance, all they hear is that Fiber=fast and this is why people question the Preact developers with stuff like "will you implement Fiber". I know that FUD wasn't the goal, but FUD is the result of overhyping a rewrite and giving it a codename.
The initial improvement that Fiber gives is about splitting up the rendering process into bite-size chunks so that the determination of what does need to change doesn't block the main thread. The rewrite of the internals also made the codebase more maintainable, and gave them a chance to implement often-requested features like returning strings or arrays from render(), as well as implementing error boundaries.