With classic ASP you usually use MSXML for XML parsing. MSXML since version 3 (which is installed anywhere where IE 6 or later is installed) supports XPath 1.0 with methods like selectNodes and selectSingleNode.
On most systems you will nowadays also find MSXML 6 as it is part of the OS since Windows XP SP 3 and Vista I think.
I guess you are already using MSXML when you say "I am currently using XMLDOM".
As for the performance issue, you could check whether MSXML 6 performs better than MSXML 3 for your queries.
Microsoft MVP (XML, Data Platform Development) 2005/04 - 2013/03