Hello Richard,
This is normal browser limitation when processing a large request of data, the application may become unresponsive or freeze as stated in this help article.
Alternative option for customization is to configure the export on the server-side as shown in this example:
http://docs.telerik.com/kendo-ui/aspnet-mvc/helpers/grid/how-to/excel-export-server-side
Don't forget to import this script when exporting the content:
<script src="//cdnjs.cloudflare.com/ajax/libs/jszip/2.4.0/jszip.min.js"></script>
Regards,
Rumen
Telerik by Progress