The first Android N developer preview is available to the public, and many users reported they felt like It was faster than the previous stable version of Android. So I decided to test this out by myself, I ran several app opening speed tests on Marshmallow on my Nexus 9 and I kept the best time I got. Then I installed the developer preview of Android N and did the same thing. (see the video below for the test)

Turns out that the developer preview isn’t faster (on Nexus 9) but just by a small margin.
What about daily use?
I noticed that the multitasking on the developer preview is much smoother than Marshmallow, and the memory management was improved: some intensive games were kept in memory on Android N. App scrolling feels a bit more sluggish on the developer preview (but that’s why It’s still a beta, almost any app isn’t optimized for it).
Demanding games feel as fluid as on Marshmallow. I tried Asphalt 8, Dead Effect 2, and Reckless Racing 3.

Conclusions: the first developer preview of Android N was surprisingly stable, leading me to think that It will be a great improvement over Marshmallow in usability and performance.