Is there a more compact way than XML to represent BPMN model?

We’re using on IE11, with Camunda as back-end, and we noticed that parsing the XML is slow when the BPMN contains many nodes. Is there a more concise representation that can be used between Camunda and (e.g. json or compact binary format)

No, there isn’t. I also doubt that importing diagrams represented as JSON would be much faster. What’s actually slow is rendering the diagram, not parsing the XML.