"add member" Pop-Up Search Takes A Long Time (ARS Webinterface)

Does anybody know how the active directory objects are retrieved with quick search and with the "add member" pop-up?

Doing a quick search is getting the objects from any unmanaged domain really fast. The result is displayed in about two seconds.

When I want to add an object to a group, choose the domain and search for the object it takes about 45 seconds until the result is displayed. This is a bit annoying as there is no notification the search is started (like a progressbar or something). Because of this the user clicks multiple times on the search button and delays the result even more.

The funny thing is: After displaying the result of the first search each consecutive search takes just two seconds.

If I'm not mistaken this is true for one unmanaged domain. Searching in other unmanaged domains does not take this long.

So can anybody tell me why this takes so long? Could there even be a misconfiguration for the unmanaged domain?