The provided information is not enough to be able to determine the reason for the slow performance. In order to be able to provide more-to-the-point answer we will need to be able to examine the specific scenario. Could you please open a support ticket and provide a sample project reproducing the problem so we can debug it locally?
In addition, to increase the performance of RadSplitter you can set its VisibleDuringInit
property to false.
the Telerik team