Reason for slow loading and render of Javascript XML import

I am working on top of open source code (code dot org) and notice that the XML import and render is extremely slow. The code it was forked from (Google blockly) is much faster. Want to fix the main performance blockers so that the code runs much faster. Can refer to the blockly code if needed to see how it is resolved in the new codebase. You can find more details on reproducing the issue here : [login to view URL] I understand that the performance hit is likely associated with the conversion of XML to DOM but want someone to identify the exact reason, compare with the newer code base if required and fix the same. Expected outcome is that the performance of the old codebase is comparable to the new one