Best comparison I've seen on the subject:
http://www.raywenderlich.com/553/how-to-chose-the-best-xml-parser-for-your-iphone-project
The difference between the slowest and fastest parsers in his case was a factor of 2. Depending on your feature requirements, there are definitely parsers that can cut your parsing time in half.
1.4m articles
1.4m replys
5 comments
57.0k users