Workaround: disable the animations this.radMultiColumnComboBox1.MultiColumnComboBoxElement.DropDownAnimationEnabled = false;
The issue is duplicated with another feedback item and both are merged into - FIX. RadMultiColumnComboBox - slow when using animations and large data sets. Here is the link: http://feedback.telerik.com/Project/154/Feedback/Details/111763-fix-radmulticolumncombobox-slow-when-using-animations-and-large-data-sets The issues are fixed in Q1 2014 (2014.1.226) version.