Fairly often, the game will freeze for several seconds. Alt/Cmd-tabbing to a different application seems to cause the game to unfreeze most of the time, but not always. It appears as if the problem is server-side, because the game stops processing events during freezes, however, the freeze also prevents the controls from doing anything and the graphics do not update. I do not know what causes the bug and therefore can not provide you with steps to reproduce it.
Please attach the complete output of the "Development console" which can be found on the second tab of the launcher.
If the launcher closes after game start, please edit your profile and select "Launcher visibility" then, "Keep the launcher open".