I was developing a Game in Unity 3.3 (iOS) using Terrain 4 Mobile and it was working good. I upgraded to 3.4 and deleted my T4M Terrain to take advantage of the announced Unity 3.4 Terrain compatibility on Xcode. I tested the Game and I lost about 40% of performance. I converted again my Terrain to T4M and everything worked fine again.
CONCLUSION: If you are using T4M and upgrade to Unity 3.4 DO NOT delete your Terrain because the Unity Terrain does not work well and your Game will lost performance.
Dreamora anticipated this behavior some time ago: Thanks Dreamora!