This is actually a problem with IE 11 that I reported.  To reproduce:

1. Create an ASP.Net page, add a RadFormDecorator 
2. Add an ASP.Net Dropdownlist, and either through data binding or code, load it with around 1400 list items
3. On the page click to open the list, and select an item.  THIS IS WHERE THE DELAY HAPPENS.  There is a long delay before the list closes with the item selected.
Sometimes, the browser gives a message about a long running script.
4.  Change the RadFormDecorator to exclude the SELECT tag.  Run the page again and notice that the delay is not there.