As promised, here is the full video of the Ninety Nine Nights demo, only available for the moment in japanese stores. This will quickly become a habit, so you'll find inside a full video in 640x360 resolution, and the most interesting (gameplay or graphics) parts of this video are also available in full 720p resolution.
And thanks again to DJmizuta for sending me the demo !
A good friend of mine, usually very well informed, told me he saw a recent version of the game running and that the slowdowns are fixed now. Let's hope he's right!