First, watch this video.
http://www.youtube.com/watch?v=Dg8hDlolN3UThis game can look that good, with that many models, run 60 FPS in full 3D, all on the 3DS.
Meanwhile X&Y have 'such a wonderful and amazing overworld' that the 3D had to be disabled just so the system could run it. And when you /can/ turn 3D on in battles, there is a very large chance it'll cause slowdown despite there being only 2 main models on screen, not including the environment.
Seriously, how did Gamefreak fuck up that badly? Every other major 3DS game can run in 3D, with most being at 60 FPS. And most of those look more impressive than X&Y. How did such a major flaw get passed quality check?