How do I optimise XPath Querying over a large dataset?
Create an XML index on the column, you will have to create a primary index and then a secondary index on the path or value you are querying. 
CREATE PRIMARY XML INDEX PXML_NAME ON SCHEMA.TABLE (XMLCOLUMN);
then create a secondary index for the { VALUE | PATH | PROPERTY } you are querying with Xpath

https://msdn.microsoft.com/en-us/library/ms191497.aspx and 
https://msdn.microsoft.com/en-us/library/bb934097.aspx