Stuck on Unity Logo? or just a long loading time in the first loading?

Couple days I asked a similar questions, but still don't know how to solve this. the situation is: 1:New build on iPad always takes very long time in loading (I need to stare at Unity logo for at least 30 seconds, if the scene bigger, it might takes couple minutes) 2.After it's successfully loaded, I turn off my app on iPad2 (>Hit home button twice >touch and hold on my app icon till a tiny red mark came out); and then I turn on my app again. This time, the loading time looks normal (Unity logo only takes about 1 second). So, just wondering, the Unity logo (scene loading) takes that long only happen to first time build process and for iPad2 only? Because for android, everything is fine.