If the application of the blur is what's taking all the time, the problem lies with ImageMagick; there is nothing you can do within Qt that will have any impact on ImageMagick performance.

You should determine where, exactly, your program is spending its time. Use of a profiler, already mentioned, will be useful. Simply dumping print statements to the console periodically ("file opened...file read...image created...image processed...etc) will also give you a feel for which steps are taking the longest.