Performance can be affected by the amount of HTML markup and scripts on the page. That includes the number of widgets and the number of their items, event handlers, etc.
A best practice is to load stuff on demand and avoid loading large quantities of data. With regard to Kendo UI, you can try the following techniques:
Generally, the performance of IE8 can never be as good as the one of new browsers, but judging by the provided information, the discussed page is heavy for all browsers and optimization may be desirable.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.