Is UIImageWriteToSavedPhotosAlbum slow in iOS SDK 4.0?

I use UIImageWriteToSavedPhotosAlbum to save picture to album. But after updated 开发者_JAVA百科to iOS SDK 4.0, I noticed that it was slowed down significantly. It's about 20 seconds, between UIImageWriteToSavedPhotosAlbum was issued and completionSelector was triggered. Can anyone help on this problem?

In my case, UIImageWriteToSavedPhotosAlbum is not stable. Sometimes it works, sometimes not.