YUI ships with a pre-combined file, yahoo-dom-event.js, because you almost always want to include these three sections of the library together. However, JIRA includes them separately:

<script type="text/javascript" src="/jira/s/330/1/1.0/_/download/resources/jira.webresources:yui/yahoo.js" ></script>
<script type="text/javascript" src="/jira/s/330/1/1.0/_/download/resources/jira.webresources:yui/dom.js" ></script>
<script type="text/javascript" src="/jira/s/330/1/1.0/_/download/resources/jira.webresources:yui/event.js" ></script>

You should change your web resources XML file to just include the single file: yahoo-dom-event.js. I think it's in the same directory in the YUI distribution as these files.

This would reduce your load time by reducing the number of requests slightly.