The problem appears to be related to the image and table resizing feature (introduced in Q3 2013), implemented for the Chrome browser. When the content has a large amount of nested tables with images, selecting an image is causing a slow operation and even an unexpected crash of the browser. 

With this example code you can disable the feature and resolve the performance issue:

<telerik:RadEditor ID="RadEditor1" runat="server">
</telerik:RadEditor>
<script type="text/javascript">
	Telerik.Web.UI.RadEditor.prototype._initializeResizableWidget =
		function myfunction() { };
</script>

Note that this line of code will disable the image/table resizing only for Chrome. Under Firefox and IE this feature is provided by the browser and will still be available.