Straight from Microsoft’s Improving .NET Application Performance and Scalability: Improving XML Performance:

Design Considerations

Check

Description

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Choose the appropriate XML class for the job.

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Consider validating large documents.

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Process large documents in chunks, if possible.

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Use streaming interfaces.

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Consider hard-coded transformations.

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Consider element and attribute name lengths.

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Consider sharing the XmlNameTable.

Parsing XML

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Description

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Use XmlTextReader to parse large XML documents.

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Use XmlValidatingReader for validation.

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Consider combining XmlReader and XmlDocument.

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On the XmlReader, use the MoveToContent and Skip methods to skip unwanted items.

Validating XML

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Description

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Use XmlValidatingReader.

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Do not validate the same document more than once.

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Consider caching the schema.

Writing XML

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Description

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Use XmlTextWriter.

XPath Queries

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Description

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Use XPathDocument to process XPath statements.

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Avoid the // operator by reducing the search scope.

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Compile both dynamic and static XPath expressions.

XSLT Processing

Check

Description

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Use XPathDocument for faster XSLT transformations.

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Consider caching compiled style sheets.

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Consider splitting complex transformations into several stages.

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Minimize the size of the output document.

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Write efficient XSLT.

Atsushi Eno from the Mono project took his time to write what he thinks the “true” Checklists: XML Performance should be. I can see how some of his (different as compared to Microsoft’s recommendation) points could be valid based on his personal experience but he confused me with his very first statement:

Avoid XML as long as possible.

Will have to email him and clarify…

UPDATE: I did email Atsushi Eno and he graceously took the time to answer some of my questions! Neat!