I guess this is caused by the default discover time.
For unknown devices, the method DiscoverDevices(int maxDevices, bool authenticated, bool remembered, bool unknown ) of BluetoothClient takes default 10 seconds, that is, the last parameter is set "true".
You can change the value by setting the property "InquiryLength".
Please see http://inthehand.com/library/html/P_InTheHand_Net_Sockets_BluetoothClient_InquiryLength.htm