So you have just bought a new PC with a multi-core CPU to replace your aging single core machine and you expect to see a huge improvement when you compile a Qt project. And you are going to be disappointed...
By default, QtCreator will only use one core at a time, you'll need to manually specify the number of cores you want to use!
To configure Qt properly you'll need to go to the project tab on the left and add -j <number of core> to the make parameters of your project as illustrated on the following picture (excuse my French!)

QtCreator multicore configuration

This solution is also mentionned somewhere on Qt's website but I think most people don't know about it.

The -j parameter in make specifies the number of job to run simultaneously. It is supposed to match the number of available cores on the system. Some say that it should be <number of core + 1> to deal with I/O delays, but I didn't observe any improvement by doing so.

When compiling the qtdemo example we can clearly see a difference in terms of CPU usage in the system monitor :

Without the -j parameter, most of the time there is only a single core used at a time:

qtdemo compiled without the -j parameter

But with the -j parameter, all the cores are used at 100% at the same time and the compilation time is much shorter:

qtdemo compiled with the -j parameter

By manually measuring with the time command, compiling qtdemo takes 29 seconds when using a single core and only 9 seconds when using 4 cores. I would have expected to have a 4x performance improvement but 3.2x faster is quite nice, everything can't scale perfectly in the real world.