After you quit the game by clicking on the red cross two java tasks from the launcher seem to be still running. They increase my CPU-Usage by at least 50 %. Due to this effect I can't play other games or Minecraft itself without laggs. The game runs still fine after you close the two tasks and laggs are reduced.
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Done. But I don't know what you mean with the command line. As you can see, there currently is only one running task (The screenshot was made after I closed Minecraft) but sometimes there are two.

Menü "Ansicht" -> "Spalten auswählen" -> "Befehlszeile"
Create the screenshot after having two running tasks with the Befehlszeile visible in the task manager
Confirmed as a problem here as well.
Launcher Java process will not exit after Minecraft is closed.
K9 is not involved. This is not a duplicate of 1408.
Please attach a screenshot of the task manager with the command line column enabled.