Amazon boasts that its fourth-gen Kindles achieve faster page turns as a result of two factors. First, it says it uses a slightly faster SoC at the heart of the device. Second, the new Kindles allow page turns without blanking the screen.
Normally, when an E Ink display changes its image, all of the microcapsules have to be unpolarized to reset the screen. That's what happens when you see the screen go completely blank during a page-turn. It's a lot like shaking an Etch A Sketch.
The new Kindles take a shortcut by unpolarizing the visible microscapsules and then polarizing only those needed to display the next image. The result is a faster page turn, yes. But it's not without a side-effect.
Without resetting all of the microscapsules, you end up with a bit of ghosting, illustrated in the picture above. The effect is slightly exaggerated due to lighting. In a dim setting, the more faded text is only slightly noticeable. In direct sunlight or a well-lit room, it's almost impossible to see.
Also, the effect is made intermittent by the Kindle's options. When you disable Page Refresh, the E Ink display still performs a complete refresh every every six or so page turns.
We haven't yet recorded any evidence that toggling the Page Refresh option improves the time it takes to complete a page turn. Every video we have from our 1000 FPS high-speed camera yields the same time difference. Hit play on both of the sequences above to see for yourself (focus on the words "singular tragedy").
There might be some difference between the Kindle Keyboard and its predecessor, but it's hard to tell. In the real-world, we consider those claims of better performance to be a wash.