Row selection becomes slower when grouping and/or hierarchy is used, because the :not CSS selector is slow in Internet Explorer and this selector is used internally by the Grid. Performance is improved in the latest Kendo UI releases, so you can consider upgrading. Still, selection will be always slower when hierarchy is used, compared to the case when it is not used. Using a smaller page size is recommended.
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI