We were recently going to upgrade from Kendo 2012Q2 to 2013Q1. Unfortunately we can't because we require support for IE8+. 

Our issue is the grid's dataItem method. In IE, it's just too slow. In order to use the newer version of Kendo, we have to upgrade to jQuery 1.9.1 (supplied in the Kendo package). In some cases, the selectors you are using are 13x slower than they were in jQuery 1.7.1.

http://jsperf.com/jquery-1-9-performance-issues

In the dataItem method, you're using the ">" selector instead of children. This, coupled with :not causes this method to crawl. 

In the jsperf test I linked above, I profile many different scenarios. You might want to take a look and see if perhaps you can make use of the .children() method and .not() instead of :not in future releases.