Application build in QtC is VERY slow

Building big project (~150 MB of sources) manually in konsole with 

time make -j8

takes only 4 min and loads all the 8 cores to 100% all the build time. 

While same project build in QtC with configured 8 cores in Project Settings load:

• on build start 8 cores partially to 40-50%,
• and after few seconds only 1 core to 100% .
• final linking also loads all the cores to 100%.

Parallel jobs set to 8 or manual -j8 switch in Make arguments does not work at all and clean build takes about 35 min! While building QtC is very slow and unresponsive: code editing and navigation, menu, buttons, dialogs reactions takes ages.