high GPU usage(131ms) when UGUI canvas is active

Hi guys,I'm working on a 2d game, it works fine on pc ,mac, android, but when I run it on iPhone4, once I clik a button to show Player's Package(an UGUI Canvas with some panels and texts),the fps drops from 31 to 5~8, here is the canvas hierarchy

most of panels in the canvas just has a text object, except the Package/Forge panel has a scroll rect object and some items(one image and on text) in it.

I camptured a GPU frame in XCode,found that when canvas is active the GPU usage become very high immediately,but cpu is low,and the fps drops to 5~8,here is the screenshot.

Why the GPU usage is so high? The report just tells me GPU cost is 131.6ms , is there any way to know some more details?(about which gameobjects spend a lot of GPU time)

and another quesiton: why the GPU report shows me a OpenGL ES error(clear mask should not be zero), how can I fix it? the error happened in GfxDeviceGLES.cpp。

sorry for my english, if the question is not very clear for you,please let me know.