In 0.11.0 development cycle Juzef did a great job identifying and resolving performance problems with our history SQL backend. He basically normalized database scheme and voila - history file is now 2 to 3 times smaller than before (with the same information!). That means that loading history into memory is 2 to 3 times faster and processing indexes is also a lot faster. So opening history window is also a lot faster than before!
Messages from history are loaded into chat windows so you can remebmer last conversation with given person. In 0.11.0 this loading is asynchronous - that means you get your window open instantly and you can send and receive messages right away. Old messages are inserted into chat window when they are ready - which can take a second or two for really big histories. In Kadu 0.10.0 you had to wait for that to happen before you could even chat.
History is imported automatically - you don't have to do anything (only wait 30 second at first run of 0.11.0). After import new history is available under ~/.kadu/history1.db file instead of old ~/.kadu/history/history.db.