I'm posting this topic just as info, if you encounter this problem and to save you the trouble of resultless Googling.
So, if you encounter a problem where the file selector windows stay on the screen for 30 seconds or longer, the problem is with a Qt config file (~/.config/QtProject.conf) which has grown to an insane size which is slowing down the whole system cuz it takes time to read the whole file - in my case it had become 480 MiB. Normally it should be 95 bytes with just 4 lines in it. So simply delete that config file and reboot the system. Logging out of session doesn't work as you hope, it has to be a full system reboot.
So, if you encounter a problem where the file selector windows stay on the screen for 30 seconds or longer, the problem is with a Qt config file (~/.config/QtProject.conf) which has grown to an insane size which is slowing down the whole system cuz it takes time to read the whole file - in my case it had become 480 MiB. Normally it should be 95 bytes with just 4 lines in it. So simply delete that config file and reboot the system. Logging out of session doesn't work as you hope, it has to be a full system reboot.