We responded to you in the support ticket about this case but for community reference I will give the response from the ticket in this post too:
We looked into the issue and indeed in that scenario there seem to be performance issues in the control.
We logged the issue in our Public Issue Tracker System where you can vote for it and track its status.
I updated your Telerik points for bringing this to our attention. We are sorry for any inconvenience that this may be causing.
the Telerik team