PyQt: user interface freezed when using concurrent.futures.ThreadPoolExecutor

[iMath]

I think the user interface shouldn't be freezed when using concurrent.futures.ThreadPoolExecutor here,as it executes asynchronously ,
 but it doesn't meet my expectations,anyone can explain why ? any other solutions here to not let user interface freezed?

code is here
http://stackoverflow.com/questions/27393533/user-interface-freezed-when-using-concurrent-futures-threadpoolexecutor