While installing files, the installer shows a progress bar.
For the stand-alone Qt 5 binary installer, which has only two components (sources and binaries), the progress bar starts at 34%, jumps to 83% when sources finish installing then to 100% when the binaries finish installing. If you only select one of these components to install, the progress bar starts at 66% and stays there throughout the entire installation until jumping to 100% at completion.
For the QtSDK installer, this problem is still present but is less noticable as there are many more components (the progress bar starts at 3%).
A more useful progress indication would start at 0% and show the percentage of the total number of files installed so far. This is what the old nsis-based Qt 4 installers do.