My experience so far is that after flashing, an N9 becomes smooth as butter, very responsive and quick. I am most of the time amazed by how snappy it is. But over time it slows down and sometimes feels sluggish up to the point of becoming unusable.
I am wondering whether that has anything to do with enabling developer mode and tweaking the device (which is part of the fun of the N9).

I've refreshed the phone again a couple of days ago, but have so far refrained from enabling developer mode and I have just installed apps that don't need any tweaking (e.g open mode, etc.). It still feels snappy. Anyone having same experiences?