如何解析一个巨大的xml文件

I need to parse a xml file which is huge in size. Its size will be little above 400-500 MB. My application is completely based upon this data. But parsing takes huge time. On iPhone it takes more then 10 minutes for every request that I make. I am using built in NSXMLParsers of iphone sdk to parse the data. What should i do now?I need to parse a xml file which is huge in siz