Launching Blizzard games plummet bandwidth for other programs.
1- Open google and Run a speed test --> test result: 30Mbps download, 5 Mbps upload
2- Launch Heroes of the storm, StarCraft II or Diablo III
3- Press CTRL + ESC to switch back to windows
4- Run Speed test again --> 0.5 Mbps download , 0.02 Mbps upload
5- Close the game
6- Run Speed test again --> 30 Mbps download , 5 Mbps upload
Note: Behavior is consistent no matter how many times I restart my machine or router, I first noticed it because of Youtube videos automatic change of quality according to speed then I decided to run the test when it looked suspicious