There are no ‘correct’ answers to generic performance questions. A proper solution can be suggested only after examining the specific use case and environment. However, a few recommendations are as follows:
If you are dealing with UNTYPED XML, try to convert them to TYPED XML. When using TYPED XML values, SQL Server has got better understanding of the structure of the XML document, data types of elements and attributes etc. This allows SQL Server to process the XML document more efficiently.
XML Indexes can be used to speed up queries that read information from XML data type columns.