Why using WWW fetch local file on IOS causing main thread blocked?

Hi,
I am working on a project and using WWW class to 'download' some assetbundle (compressed) on local device. Something strange is when i run it on PC or Android device all things are working fine, but on IOS platform it seems to take a long time to fetch the local file and cause main thread blocked.

some code snapshot like:

Code (CSharp):

1. //.....
2. WWW donwloader = new WWW("file://....");
3. yield return downloader;
4. //...

on my iphone6 (ios 8.4) all things are block after yield and resume after www finished its work.
why on ios?

Unity using 4.6. 7p3

 

eUh....

It seems WWW implementation on IOS is different?