Eclipse Mars is a great IDE which a lot of performance improvements against older versions of Eclipse. But I still have in some workspaces the problem that my Eclipse IDE hangs during validation of XML files of my Web Projects (e.g. web.xml or faces-config.xml). This problem is well known for the Eclipse IDE. To solve this problem in Eclipse Mars it is sufficient to disable the Validation of “XML Schema Files”.
and also disable the Option “Honour all XML schema location” for the XML Files validation.
If you disable this options, Eclipse will be faster after updating xml files.