Even small XML files are slow to open.
If the XML file references external XML Schemas, then these Schemas are also loaded in order to provide validation and intellisense, if the schemas are located on a remote server then this can take quite a long time.
External schema may be picked up if the namespaces in the XML file match ones in the XML Schema Library
, or if it contains specific references to schemas using the schemaLocation or noNamespaceSchemaLocation attributes.
Solution - Caching the schema files.
If the XML Schema files you are using are static (i.e. they don't often change), then you can add them to the cache, they are then stored locally and are much faster to load next time.
If the schema files change you can purge the files from the cache manually.