Are you doing the parsing within a browser, loading the XML from a server? In that case, I don't think SAX is a reasonable option. You need to reduce the size of the data you are downloading, doing more of the work on the server.
To help with application architecture decisions one really needs more than a two-line requirements statement.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference