Validation of XML files in Eclipse is slow because the schema files are downloaded over and over again. This is because the server returns the no-cache header. I think there is no reason for this.

Examples of affected urls:
http://jboss.org/j2ee/dtd/service-ref_4_0.dtd
http://jboss.org/schema/arquillian/arquillian_1_0.xsd
http://jboss.org/ironjacamar/schema/ironjacamar_1_0.xsd
http://docs.jboss.org/ironjacamar/schema/datasources_1_0.xsd

I don't have a definitive list, perhaps rob.stryker could help here.