I had a similar problem, I tracked down the problem using the debugger - I could see the program start up quickly and as I stepped through the creation of the classes, I noticed a 5 second delay on one of the classes. That class was searching for a configuration file and every time it attempted to open the file, Windows blocked execution for 5 seconds before returning with the error code.

For your case, I'd make use of the qDebug() statement, you may find that your program is starting quickly, but doing some time intensive work before showing the main window, which makes it look like its slow to startup.