I have a grid with around 20,000 items in it and when I click export to excel with .AllPages set to true, I get an unresponsive script error. I've traced the SQL query and that is quick, taking only 1 second to complete but I can see the ajax call taking far too long and ultimately failing after around 15-20 seconds. What are my options for this? Is there an alternative approach involving server-side?
I would guess this is because there are a lot of items in the grid and your example on your demo page only does the current page ?

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